FINAL ANSWER:
A. The thermostat calls for heat.
Step-by-step explanation:
In the context of heating systems, the first step in the operational sequence is initiated when the thermostat calls for heat. When the thermostat senses that the temperature in the area it controls has fallen below the desired temperature setting, it sends a signal to the heating system to start heating the space. This signal is what triggers the heating system to begin its operation.
The other options listed describe subsequent steps in the heating system's operation:
- OB. The burners light: This typically occurs after the thermostat calls for heat, as the burners are activated to generate heat.
- OC. The draft switch closes: The draft switch is a safety feature in some heating systems and closes as a part of the system's safety checks and startup sequence.
- OD. The igniter is energized: The igniter is activated to ignite the fuel (e.g., gas or oil) in the heating system, but this step comes after the thermostat calls for heat.
So, the first and initial step is when the thermostat signals the need for heat.