带头结点的单链表first为空的判定条件是（）。

A:first==NULL B:first->1ink==NULL C:first->link==first D:first!＝NULL

The (first) Olympic (Games) (in) modern (time) took place in 1896.

A:first B:Games C:in D:time

A pet is an animal that (21) in your house. Over half the families in North America own pets. The most popular (22) are dogs, fish and birds. Rabbits, hamsters and turtles are also popular. Even some unpopular animals such as frogs, mice and snakes are pets.
People first (23) to keep pets about 12,000 years ago. The first pet was a dog. The dog did not become a pet because people (24) it was cute. It became a pet because it was useful. People used the dog for many things. The dog warned people (25) danger so wild animals didn’t attack people. It also ate left-over food. Later, dogs helped people take care (26) cows and sheep on farms.
Cats did not (27) pets for a long time. When the Egyptians first started to grow grain, mice ate it. The Egyptians were (28) to tame cats.
Today, people keep many (29) of pets. Animals such as canaries, parrots, mice and hamsters live in cages. They need people to bring them food and water and to keep their cages clean. Other animals, such as dogs and cats, need food and water, but they can take care of themselves (30) .They are good pets for people because they don’t need much care.

A:first B:firstly C:at the first D:the first

A pet is an animal that (21) in your house. Over half the families in North America own pets. The most popular (22) axe dogs, fish and birds. Rabbits, hamsters and turtles are also popular. (23) some unpopular animals such as frogs, mice and snakes are pets.
People first (24) to keep pets about 12,000 years ago. The first pet was a dog. The dog (25) not become a pet because people (26) it was cute. It (27) a pet because it was useful. People used the dog for many things. The dog warned people (28) danger so wild animals didn’t attack people. It also ate leftover food. Later, dogs helped people take care (29) cows and sheep on farms.
Cats did not (30) pets for a long time. When the Egyptians first (31) to grow grain, mice ate it. The Egyptians were (32) to tame cats.
Today, people keep many (33) of pets. Animals such as canaries, parrots, mice and hamsters live in cages. They need people to bring them food and water and to keep their cages clean. Other animals, such as dogs and cats, need food and water, but they (34) take care of themselves (35) . They are good pets for people because they don’t need much care.

A:first B:firstly C:at the first D:the first

(The metal) aluminum (has been) (first) isolated (early) in the nineteenth century.

A:The metal B:has been C:first D:early

A pet is an animal that (21) in your house. Over half the families in North America own pets. The most popular (22) are dogs, fish and birds. Rabbits, hamsters and turtles are also popular. Even some unpopular animals such as frogs, mice and snakes are pets.
People first (23) to keep pets about 12,000 years ago. The first pet was a dog. The dog did not become a pet because people (24) it was cute. It became a pet because it was useful. People used the dog for many things. The dog warned people (25) danger so wild animals didn’t attack people. It also ate left-over food. Later, dogs helped people take care (26) cows and sheep on farms.
Cats did not (27) pets for a long time. When the Egyptians first started to grow grain, mice ate it. The Egyptians were (28) to tame cats.
Today, people keep many (29) of pets. Animals such as canaries, parrots, mice and hamsters live in cages. They need people to bring them food and water and to keep their cages clean. Other animals, such as dogs and cats, need food and water, but they can take care of themselves (30) .They are good pets for people because they don’t need much care.

A:first B:firstly C:at the first D:the first

The (first) Olympic (Games) (in) modern (time) took place in 1896.

A:first B:Games C:in D:time

Directions: For each blank in the following passage, there are four choices marked A, B, C and D. Choose the one that is most suitable and mark your answer by blackening the corresponding letter on ANSWER SHEET I.

A:first B:firstly C:at the first D:the first