超声仪器分类中，"以脉冲幅度高低表示回声强弱，以振幅"Amplitude"词首A命名"属于（）。

A:A型超声仪 B:B型超声仪 C:M型超声仪 D:D型超声仪 E:彩色多普勒血流显像仪

数字调幅又称为（）移键控ASK（Amplitude Shift Keying）.

潮汐（tide）

潮汐周日不等（diurnal inequality of tide）的现象为（）。

A:一天有两次涨潮和两次落潮 B:相邻两次高潮或两次低潮潮高不等 C:涨落潮时间不相等 D:一天有两次高潮或两次低潮的潮高以及相邻的高、低潮的时间间隔不相等

大潮（spring tide）与小潮（neap tide）

There are many reasons of the tidal action which is a very complicated phenomenon involving many factors all of which must be considered to predict tide in any one place at any one time. The alternate rise and fall of the surface of the seas and the ocean is mainly caused by the attractive force of the moon alone. When the attraction is great, the tide is high; and conversely when the attraction is minimal, the tide is low. Between the high and the low tide is a period of a little over six hours, being called during its rise a flood tide, and during its fall an ebb tide.
The sun also exerts its effect on the tide, being a lit fie less than half as much as the moon, although its mass is so much greater. When the moon and the sun are horizontal, the highest high tide occurs and when they are vertical the lowest low tide takes place.
The wind also plays an important part in the modification of the normal rise and fall of the tide. Thus these factors affect each other, either detracting or adding to the attractive forces to add or subtract from the weight of the tide. But the tides still move inexorably on, rising twice a day, getting a little later each day and posing a problem of correctly predicting their heights and lows.

A:7:20 Ａ.M. B:B．8:00 Ａ.M. C:C．1:45 P.M. D:D．high noon

The power produced by the waves, tide and wind ______.

A:is not as consistent as it should be B:is generally less in winter than in summer C:is never likely to be very great D:is consistent

If the first high tide takes place at 1:00 A.M., the next high tide will probably take place at ______.

A:7:20 Ａ.M. B:8:00 Ａ.M. C:1:45 P.M. D:high noon

Which of the following factors will not affect the time of the tide

A:The position of the sun and the moon. B:The mass of the moon. C:The wind. D:All of the above mentioned factors.

The people of Kiribati worry that one day their country will be taken away by a sudden high tide.

A:A．