单选题The mechanical efficiency of an engine （） because losses occur in the form of friction.Ais always less than 100%Bis always more than 100%Cis always equal to 90%Dis always less than 90%
单选题
The mechanical efficiency of an engine （） because losses occur in the form of friction.
A
is always less than 100%
B
is always more than 100%
C
is always equal to 90%
D
is always less than 90%
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解析：
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