单项选择题
A.The cable lifter
B.The hoisting motor
C.The topping wire
D.The life-raft
单项选择题 Topping wires fix the height of the derricks and （） to the deck may be used to prevent fore and aft movement.
单项选择题 Large pipes or down-comers are fitted between the steam drum and （） to ensure good natural circulation of water.
单项选择题 With reference to a rotary vane steering gear system, which one of the following statements is true?（）