单项选择题
A.starting air pressure too high
B.correct timing of starting air valves
C.three of the starting valves stuck
D.the turning gear disengaged
单项选择题 When turning the engine with turning gear, you should keep the indicator cocks （）.
单项选择题 When to remove scale from the cooling spaces, acid solution （） in the system can be tested from time to time by putting some on to a piece of lime.
单项选择题 When the water produced from most evaporator is supplied for drinking purpose, it must be treated to ensure it is biologically pure and （）.