Non-indigenous (non-native) species of plants and animals arrive by way of two general types of pathways. First, species having origins outside the United States may enter the country and become established either as free-living populations or under human cultivation-for example, in agriculture, horticulture, aquaculture, or as pets. Some cultivated species subsequently escape or are released and also become established as free-living populations. Second, species of either U.S. or foreign origin and already within the United States may spread to new locales. Pathways of both types include intentional as well as unintentional species transfers. Rates of species movement driven by human transformations of natural environments as well as by human mobility-through commerce, tourism, and travel-greatly exceed natural rates by comparison. While geographic distributions of species naturally expand or contract over historical time intervals (tens to hundreds of years), species’’ ranges rarely expand thousands of miles or across physical barriers such as oceans or mountains. 　　Habitat modification can create conditions favorable to the establishment of non-indigenous species. Soil disturbed in construction and agriculture is open for colonization by non-indigenous weeds, which in turn may provide habitats for the non-indigenous insects that evolved with them. Human-generated changes in fire frequency, grazing intensity, as well as soil stability and nutrient levels similarly facilitate the spread and establishment of non-indigenous plants. When human changes to natural environments span large geographical areas, they effectively create passages for species movement between previously isolated locales. The rapid spread of the Russian wheat aphid to fifteen states in just two years following its 1986 arrival has been attributed in part to the prevalence of alternative host plants that are available when wheat is not. Many of these are non- indigenous grasses recommended for planting on the forty million or more acres enrolled in the U.S. Department of Agriculture Conservation Reserve Program. 　　A number of factors perplex quantitative evaluation of the relative importance of various entry pathways. Time lags often occur between establishment of non-indigenous species and their detection, and tracing the pathway for a long-established species is difficult. Experts estimate that non-indigenous weeds are usually detected only after having been in the country for thirty years or having spread to at least ten thousand acres. In addition, federal port inspection, although a major source of information on non-indigenous species pathways, especially for agriculture pests, provides data only when such species enter via closely-examined routes. Finally, some comparisons between pathways defy quantitative analysis-for example, which is more "important": the entry path of one very harmful species or one by which many but less harmful species enter the country Which of the following may best express the chief purpose of the last paragraph.’’

A:To explain the difficulties in tracing the pathways for long-established species. B:To describe the events leading to the detection of non-indigenous species. C:To identify the problems in assessing the weight of entry tracks for foreign species. D:To discuss the role of time lags and geographic expansion in species detection.

Non-indigenous (non-native) species of plants and animals arrive by way of two general types of pathways. First, species having origins outside the United States may enter the country and become established either as free-living populations or under human cultivation-for example, in agriculture, horticulture, aquaculture, or as pets. Some cultivated species subsequently escape or are released and also become established as free-living populations. Second, species of either U.S. or foreign origin and already within the United States may spread to new locales. Pathways of both types include intentional as well as unintentional species transfers. Rates of species movement driven by human transformations of natural environments as well as by human mobility-through commerce, tourism, and travel-greatly exceed natural rates by comparison. While geographic distributions of species naturally expand or contract over historical time intervals (tens to hundreds of years), species’’ ranges rarely expand thousands of miles or across physical barriers such as oceans or mountains. 　　Habitat modification can create conditions favorable to the establishment of non-indigenous species. Soil disturbed in construction and agriculture is open for colonization by non-indigenous weeds, which in turn may provide habitats for the non-indigenous insects that evolved with them. Human-generated changes in fire frequency, grazing intensity, as well as soil stability and nutrient levels similarly facilitate the spread and establishment of non-indigenous plants. When human changes to natural environments span large geographical areas, they effectively create passages for species movement between previously isolated locales. The rapid spread of the Russian wheat aphid to fifteen states in just two years following its 1986 arrival has been attributed in part to the prevalence of alternative host plants that are available when wheat is not. Many of these are non- indigenous grasses recommended for planting on the forty million or more acres enrolled in the U.S. Department of Agriculture Conservation Reserve Program. 　　A number of factors perplex quantitative evaluation of the relative importance of various entry pathways. Time lags often occur between establishment of non-indigenous species and their detection, and tracing the pathway for a long-established species is difficult. Experts estimate that non-indigenous weeds are usually detected only after having been in the country for thirty years or having spread to at least ten thousand acres. In addition, federal port inspection, although a major source of information on non-indigenous species pathways, especially for agriculture pests, provides data only when such species enter via closely-examined routes. Finally, some comparisons between pathways defy quantitative analysis-for example, which is more "important": the entry path of one very harmful species or one by which many but less harmful species enter the country Which of the following may best express the chief purpose of the last paragraph.’’

A:To explain the difficulties in tracing the pathways for long-established species. B:To describe the events leading to the detection of non-indigenous species. C:To identify the problems in assessing the weight of entry tracks for foreign species. D:To discuss the role of time lags and geographic expansion in species detection.

Non-indigenous (non-native) species of plants and animals arrive by way of two general types of pathways. First, species having origins outside the United States may enter the country and become established either as free-living populations or under human cultivation-for example, in agriculture, horticulture, aquaculture, or as pets. Some cultivated species subsequently escape or are released and also become established as free-living populations. Second, species of either U.S. or foreign origin and already within the United States may spread to new locales. Pathways of both types include intentional as well as unintentional species transfers. Rates of species movement driven by human transformations of natural environments as well as by human mobility-through commerce, tourism, and travel-greatly exceed natural rates by comparison. While geographic distributions of species naturally expand or contract over historical time intervals (tens to hundreds of years), species’’ ranges rarely expand thousands of miles or across physical barriers such as oceans or mountains. 　　Habitat modification can create conditions favorable to the establishment of non-indigenous species. Soil disturbed in construction and agriculture is open for colonization by non-indigenous weeds, which in turn may provide habitats for the non-indigenous insects that evolved with them. Human-generated changes in fire frequency, grazing intensity, as well as soil stability and nutrient levels similarly facilitate the spread and establishment of non-indigenous plants. When human changes to natural environments span large geographical areas, they effectively create passages for species movement between previously isolated locales. The rapid spread of the Russian wheat aphid to fifteen states in just two years following its 1986 arrival has been attributed in part to the prevalence of alternative host plants that are available when wheat is not. Many of these are non- indigenous grasses recommended for planting on the forty million or more acres enrolled in the U.S. Department of Agriculture Conservation Reserve Program. 　　A number of factors perplex quantitative evaluation of the relative importance of various entry pathways. Time lags often occur between establishment of non-indigenous species and their detection, and tracing the pathway for a long-established species is difficult. Experts estimate that non-indigenous weeds are usually detected only after having been in the country for thirty years or having spread to at least ten thousand acres. In addition, federal port inspection, although a major source of information on non-indigenous species pathways, especially for agriculture pests, provides data only when such species enter via closely-examined routes. Finally, some comparisons between pathways defy quantitative analysis-for example, which is more "important": the entry path of one very harmful species or one by which many but less harmful species enter the country Which of the following may best express the chief purpose of the last paragraph.’’

A:To explain the difficulties in tracing the pathways for long-established species. B:To describe the events leading to the detection of non-indigenous species. C:To identify the problems in assessing the weight of entry tracks for foreign species. D:To discuss the role of time lags and geographic expansion in species detection.

Scientists have developed a slimming (减肥) drug that successfully suppresses appetite and results in a dramatic loss of weight without any apparent ill effects.
The drug interferes with appetite control and prevents the build-up of fatty tissue. Laboratory mice given the drug lost up to a third of their total body weight.
Within 20 minutes of being given the drug, called C75, the mice lost interest in eating and survived apparently content on just 10 percent of the food they would normally eat.
As a result, mice taking the drug lost 45 percent more weight than mice fed the same amount of food, which compensate for the lack of food by becoming more lazy.
Scientists, from the Johns Hopkins University in Baltimore, said that C75 is likely to produce a similar effect on humans because appetite control in brain is thought to be based on the, same chemical pathways as those in mice.
"We are not claiming to have found a fabled(神奇的) weight-loss drug. What we have found, using C75, is a major pathway in the brain that body uses naturally in regulating appetite at least in mice," said Francis Khaddar, a pathologist and senior team member.

A:mice and humans have similar brain structure B:humans are likely to be more lazy than mice C:human’s appetite is regulated through the same pathway as mice’s D:mice and human body use different pathways of appetite control

An internet is a combination of networks connected by (26) ．When a datagram goes from a source to a (27) ，it will probably pass many (28) until it reaches the router attached to the destination network．A router receives a (29) from a network and passes it to another network．A router is usually attached to several networks．When it receives a packet，to which network should it pass the packet The decision is based on optimization：Which of the available (30) is the optimum pathway

A:pathways B:routers C:diagrams D:calls