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When you test the tractor protection valve, the red air supply control knob should go into the ________ position.

A emergency
Explaination
To test the tractor protection valve, shut the engine off and step on and off the brake pedal several times to reduce the air pressure in the tanks. When the air pressure has fallen enough (typically into the range 20–45 psi), the trailer air supply control knob should pop out, or the valve control should go from the “normal” to the “emergency” position.
B normal
C neutral