单项选择题
A.fire-tubes
B.furnaces
C.water drums
D.water wall tubes
单项选择题 With reference to a rotary vane steering gear system, which one of the following statements is true?（）
单项选择题 With reference to rules governing steering gear, which one of the following rules is incorrect? （）
单项选择题 With reference to operating a four-ram steering gear system under emergency conditions, which one of the following statements is incorrect?（）