单项选择题 A man was sighted as he fell overboard. After completing a Williamson turn，the man is not sighted. What type of search should be conducted?（）

单项选择题 （）the visibility to reduce to one mile by heavy rain，it（）be impossible to rescue the vessel in distress.

单项选择题 If more than one raft is manned after the vessel has sunk，you should（）.