On a four-stroke diesel engine, when the piston is driven downwards by the expanding gas, it delivers power to the crankshaft through the （）.

A.a cooling system
B.a lubrication oil system
C.an electric system
D.an injection system

A.much
B.more
C.most
D.less

A.lower than；can lubricate
B.lower than；can cool
C.higher than；can enter
D.higher than ；cannot enter