单项选择题
James Watt （）the steam engine in 1796.
A.discovered
B.had discovered
C.invented
D.had invented
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Energy loses （）friction occurs in every engine.
A.because
B.thanks to
C.dues to
D.owing to -
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The mechanical efficiency of an engine is always less than 100% （） losses occurring in the form friction.
A.because
B.since
C.due to
D.owing to -
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The pressure difference before and after the lube oil filter indicates （）the filter should be cleaned.
A.when
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