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  3. There are many reasons of the tidal action which is a very complicated phenomenon involving many factors all of which must be co

There are many reasons of the tidal action which is a very complicated phenomenon involving many factors all of which must be co

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问题     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.
The principal idea of this article is ______.
选项 A、the attraction of the sun and the moon
B、the sun, the moon and the wind
C、the changing tide
D、the factors affecting the tide
答案D
解析 本题为主旨归纳题。本文第一句话就总结性地指出多种因素可以影响到潮水活动，接着分别指出了三个因素。
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