Answer :
Final answer:
The low-pressure alarm on large buses usually goes off at 60 psi as specified by the FMCSA to ensure enough air pressure is available for effective braking.
Explanation:
On large buses, the common industry standard for the low-pressure alarm is to activate when the air pressure in the brake system falls to a certain level. This alert is critical for the safety of the vehicle as it indicates that the air pressure may be insufficient to effectively stop the bus. The Federal Motor Carrier Safety Administration (FMCSA) regulations typically specify that the alarm should activate before the air pressure drops below 60 pounds per square inch (psi). Therefore, for the provided options, the correct answer to when the low-pressure alarm typically goes off on large buses is 60 psi.
