What is the speed of the crankshaft in a four-stroke/cycle engine when the camshaft is turning at 750 rpm?（）

A.raise lube oil viscosity for easier starting in cold weather
B.increase air inlet temperature
C.increase compression ratio
D.increase jacket water temperature for easier starting in cold weather

A.reduce engine exhaust noise
B.reduce engine back pressure
C.increase engine cycle efficiency
D.increase turbocharger efficiency

A.create turbulence in the combustion chamber
B.aid in completing cylinder scavenging
C.inject fuel at the proper time
D.minimize fuel penetration into the cylinder