单项选择题
A.use the float-free method only B.lower all boats C.raise all boats D.drill is over，secure all boats
A.Only when there is a chance of their being seen by rescue vessels B.At half-hour intervals C.At one-hour intervals D.Immediately upon abandoning the vessel
A.before the boat is lowered from the stowed position B.as the boat-fall blocks break clear of the davit head C.before the boat is lowered from the embarkation level D.after the boat is released into the water