單項選擇題Cooling makes the engine metals their mechanical properties（）.

A.Remain
B.to remain
C.retain 
D.to retain


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1.單項選擇題Gas turbines differ from steam turbines in that （）.

A.steam rather than gas is used to turn a shaft
B.vapor rather than gas is used to turn a shaft
C.the former uses gas to turn a shaft
D.the latter uses gas to turn a shaft

2.單項選擇題The reason why more and more of the large merchant vessels are being powered by medium-speed diesel engines is （）.

A.they operate between 150 and 450 rpm
B.they are connected to the propeller by gearing
C.their smaller size and weight
D.they can be connected directly to the propeller without gearing