Given: While checking the operation of a furnace, the thermostat is moved to its highest (warmest) setting to make the furnace operate. 10 seconds after the thermostat & contacts close, the burners ignite, causing a loud noise and flames to shoot out the front. Which of the following is the cause?

Given: While checking the operation of a furnace, the thermostat is moved to its highest (warmest) setting to make the furnace operate. 10 seconds after the thermostat & contacts close, the burners ignite, causing a loud noise and flames to shoot out the front. Which of the following is the cause?