121. Which of the following actions takes place in the control circuit of an automatically fired auxiliary boiler when the desired steam pressure is obtained?

A. A temperature sensing device opens the circuit breaker in the burner motor.
B. The high limit control secures power to the entire oil firing system.
C. The stack relay actuates the low limit control which breaks the ignition circuit.
D. The stack relay secures power to the high voltage side of the ignition transformer.