单选题In using reduction gears to obtain efficient propeller speeds, （）.Athey must be located at the after end of the engineBthey can only be used with one engine at a timeCthey eliminate the need for controllable pitch propellersDthey are connected to the engine with a flexible coupling
单选题
In using reduction gears to obtain efficient propeller speeds, （）.
A
they must be located at the after end of the engine
B
they can only be used with one engine at a time
C
they eliminate the need for controllable pitch propellers
D
they are connected to the engine with a flexible coupling
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