单项选择题
James Watt （）the steam engine in 1796.
A.discovered B.had discovered C.invented D.had invented
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.
单项选择题 The pressure difference before and after the lube oil filter indicates （）the filter should be cleaned.
