单项选择题
A flashing green light displayed at a single lock means that the lock is（）.
A.ready for entrance B.ready for entrance，but gates cannot be closed completely C.being made ready for entrance D.not ready for entrance
单项选择题 You are on an ice-reinforced vessel about to enter packice. You should（）.
单项选择题 Which will most likely occur when entering shallow water? （）
单项选择题 The principal danger from ice collecting on a vessel is the（）.
