单选题The film of oil between the rings and the liner is not only essential for the lubrication but also （） achieving a gas seal.A play a great part onB run a role inC plays an important part inD acts as important part at

Diesel engine blow-by is the leakage of combustion gases past the( ).

A.oil rings only

B.compression and scraper rings

C.cylinder liner sealing

D.cylinder liner ring grooves

材料：

Spark erosion is caused by a voltage discharge between the main bearing and journal surface. The cause of the potential is the development of a galvanic element between the ship’s hull, sea water, and the propeller shaft/crankshaft.

The oil film acts as a dielectric. The puncture voltage in the bearing depends on the thickness of the oil film. With increasing engine ratings, the specific load in the main bearing is increased. his will reduce the oil film thickness, and enable the discharge to take place at a lower voltage level.

Since the hydrodynamic oil film thickness varies through a rotation cycle, the discharge will take place at roughly the same instant during each rotation cycle, i.e. when the film thickness is at its minimum. The roughening will accordingly be concentrated in certain areas on the journal surface. In the early stages, the roughened areas can resemble pitting erosion. but later, as the roughness increases, the small craters will scrape off and pick up white metal-hence the silvery white appearance.

Therefore, to ensure protection against spark erosion, the potential level must be kept at maximum 80 mV, which is feasible today with a high efficiency earthing device. If an earthing device is installed, its effectiveness must be checked regularly. Spark erosion is only observed in main bearings and main bearing journals.

问题：

According to the paragraph one, a voltage between the main bearing and journal surface is produced by______.

A.propeller rotation

B.leakage from M/E electric control unit

C.a galvanic element

D.friction between the main bearing and journal surface

______engine ratings increase, the hydrodynamic oil film between the main bearing and journal surface is______.A.The lower/thinner

B.The higher/the thicker

C.The lower/thicker

D.The higher/thinner

When will the spark erosion take place on the journal surface during each rotation cycle?A.when the film thickness is at its minimum

B.when the film thickness is at its maximum

C.when the engine is on its compression stroke

D.when the engine is on its exhaust stroke

During the engine room watch keeping, which equipment must be noted to ensure protection against spark erosion?A.the antifouling plant

B.the earthing device

C.the main engine electric heating coils

D.the shaft generator

请帮忙给出每个问题的正确答案和分析，谢谢！