单项选择题
A.clockwise around the North Atlantic and North Pacific Oceans
B.clockwise or counterclockwise depending on whether it is warm or cold current
C.counterclockwise except in the Gulf Stream
D.counterclockwise around the North Atlantic and North Pacific Oceans
单项选择题 In some river mouths and estuaries the incoming high-tide wave crest overtakes the preceding low-tide trough. This results in a wall of water proceeding upstream，and is called a（）.
单项选择题 In some parts of the world there is often a slight fall in tide during the middle of the high water period. The effect is to create a longer period of stand at higher water. This special feature is called a(n)（）.
单项选择题 In shallow water，waves that are too steep to be stable，causing the crests to move forward faster than the rest of the wave，are called（）.