There are many reasons for tidal action; thus, tides are very complicated phenomenon invol
The sun also exerts its effect on the tide, being a little 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 tide occurs and when they are vertical the lowest tide takes place.
The wind also plays an important part in the modification of the normal rise and fall of the tides. Thus these factors affect each other, either detracting(转移) or adding to the attractive forces to add or subtract from the magnitude of the tide. One thing is certain: the tides move inexorably(不可阻挡) on, rising twice a day, shifting a little in time each day, and coutinually posing the problem of how to correctly predict their highs and lows.
The principal idea of this article is ______.
A.the attraction of the sun and the moon
B.the sun, the moon and the wind
C.the changing tides
D.the factors affecting the tides