单项选择题
A snag or other underwater obstruction may form a（）.
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 shipmate suffers a heart attack and stops breathing. You must（）.
单项选择题 A shipmate chokes suddenly，cannot speak，and starts to turn blue. You should（）.
单项选择题 A ship must have on board a first-aid kit that is approved by the（）.
