Cooling makes the engine metals their mechanical properties（）.

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

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

A.both of them are ignited by compressed air
B.both of them are the forms of external combustion engines
C.both of them have spark plugs
D.the power is developed by the piston in the cylinder