One of the differences between a two-stroke engine and a four-stroke engine is,（）

单项选择题One of the differences between a two-stroke engine and a four-stroke engine is,（）

A.a two-stroke engine works without exhaust operation
B.a two-stroke engine works without compression stroke
C.a two-stroke engine works without expansion stroke
D.a two-stroke engine works without suction operation

单项选择题In a diesel engine,（） works as working medium and changes the heat energy to mechanical energy, changes the reciprocating movement to rotary moment（）

A.fuel spray； the burning of the fuel
B.the burned gas mixture； connecting rod
C.medium shaft；crankshaft
D.the burned gas mixture； piston rod

单项选择题In a diesel engine, offers the heat energy directly（）； works as working medium and changes the heat energy to mechanical energy（）

A.the burning of the fuel； medium shaft
B.fuel spray； fresh air
C.fuel； fuel spray
D.the burning of the fuel； the burned gas mixture

单项选择题A propeller, in order to operate efficiently, must rotate at a relatively （） speed.

A.high
B.low
C.fast
D.lowly

单项选择题Why a gearbox is needed in a ship driven by a medium-speed diesel engine?（）

A.To reduce the main engine speed
B.To increase the main engine speed
C.To govern the main engine speed
D.To fix the propeller shaft