Question

a house is a system. the furnace is turned on. which of the following is the output? the front door closes; the doorbell rings; a person enters the house; the house heats up

Explanation:

The furnace's function is to heat the house. When the furnace is turned on, the output related to the furnace's operation should be the house heating up. The front door closing, doorbell ringing, or a person entering are not related to the furnace's operation.

Answer:

The house heats up