The usual method of unloading a low pressure air compressor at start-up is accomplished by __________.

Correct Answer: Option D — holding the suction valve open

Holding the suction valve open is the standard method for unloading a reciprocating air compressor. By keeping the valve open during the compression stroke, air is simply pushed back into the suction manifold rather than being compressed. This effectively eliminates the pressure differential across the piston, minimizing the load on the motor.