What are the four functions of the trailer spring parking brake valve?
A) Applies brakes, releases brakes, regulates air pressure, isolates trailer
B) Modulates brake pressure, controls trailer brakes, releases parking brakes, isolates trailer
C) Applies parking brakes, releases trailer brakes, regulates air pressure, isolates trailer
D) Applies service brakes, controls trailer brakes, releases parking brakes, isolates trailer