单项选择题 （） unit is mounted so as to raise and lower the cable from the spurling pipe, which is at the top and center of the chain or cable locker.

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.