单项选择题

Wristpin bearings are difficult to lubricate because of their oscillating motion and（）

A.their free-floating design
B.their relatively small size
C.the reciprocating motion of the piston
D.their position in the lubrication system

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船舶电气与船舶安全

Wristpin bearings are difficult to lubricate because of their oscillating motion and（）