Scale and dirt accumulation in the waterside of a lube oil cooler will be indicated by a gradual increase in the lube oil（）.

A.land
B.skirt
C.crown
D.plate

A.cylinder exhaust valve
B.air starting valve
C.air inlet valve
D.cylinder relief valve

A.crankshaft revolution
B.mechanical cycle of operation
C.period of two combustion cycles
D.cycle of two events