The crank pin journal is provided with large bores for （）.

A.can be recovered
B.is only supplied to cylinders
C.only feeds the main bearing
D.is a kind of detergent

A.pressure gauges
B.stretch gauges
C.thermometers
D.bridge gauges

A.should be gradually increased
B.should be gradually reduced
C.should be constantly kept
D.should be varied up and down