单项选择题
A.V-shaped ripple with the point of the V pointing upstream
B.V-shaped ripple with the point of the V pointing downstream
C.small patch of smooth water on a windy day
D.smoothing out of the vessel's wake
单项选择题 A section of the river that is narrower than usual and is often navigable from bank to bank is a（）.
单项选择题 A rotary current sets through all directions of the compass. The time it takes to complete one of these cycles is approximately（）.
单项选择题 A light，feathery deposit of ice caused by the sublimation of water vapor directly into the crystalline form，on objects whose temperatures are below freezing，is called（）.