When a FMVSS-121 compliant tractor-trailer is running down a highway and the driver applies the service brakes, which of the following is true at the time of the brake application?
A. only the service application circuit is charged with air
B. only the hold-off circuit is charged is charged with air
C. both service application and hold-off circuits are charged with air
D. hold-off circuit is exhausted and service application circuit is charged with air
Answer: C
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