单项选择题
A flashing red light displayed at a single lock means that the lock（）.
A.is ready to use but vessels must stand clear B.is ready to use and vessels may approach C.cannot be made ready immediately and vessels shall stand clear D.cannot be made ready immediately but vessels may approach
单项选择题 A flashing green light displayed at a single lock means that the lock is（）.
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? （）
