单项选择题
There shall be two principal （） through the harbor for the passage of vessels of over 60 tons burden．
A.straits B.channels C.canals D.roadsteads
单项选择题 The transfer procedures for oil products are required to be posted （）.
单项选择题 The step of a pilot ladder which prevents the ladder from twisting is the （）.
单项选择题 THE SPEED OF A VESSEL ADJUSTED TO THAT OF A PILOT BOAT AT WHICH THE PILOT CAN SAFELY EMBARK is （）.
