If the engine does start on air, but combustion does not immediately begin, the cause may not be （）.

A.failure of the fuel pumps
B.starting air pressure too low
C.stop valve in air line closed
D.the turning gear still engaged

A.low compression of the fuel pumps
B.incorrect timing of fuel pumps
C.the starting air valves stuck
D.air-locks in fuel oil pipeline

A.must to stand inoperative
B.has to stand inoperative
C.must be stood idle
D.will stand idle