单项选择题
Pump up the starting air reservoirs to their maximum pressure and （）the air reservoirs and air system.
A.open B.close C.drain D.vent
单项选择题 James Watt （）the steam engine in 1796.
单项选择题 Energy loses （）friction occurs in every engine.
单项选择题 The mechanical efficiency of an engine is always less than 100% （） losses occurring in the form friction.
