Question

what allows the bimetallic coil to turn on or off a heating or cooling system?
the two metals contract the same amount.
the thermostat is set at a particular temperature.
the two metals expand differently.
the coil comes in contact with air.

Explanation:

A bimetallic coil consists of two different metals bonded together. When heated, different metals expand at different rates. This differential expansion causes the coil to bend. When the coil bends enough, it can make or break an electrical connection, thus turning on or off a heating or cooling system. If the two metals contracted the same amount (first option), there would be no bending. The thermostat being set at a particular temperature (second option) is more about the overall function of the thermostat system but not the direct mechanism of the bimetallic coil's action. The coil coming in contact with air (fourth option) is too vague and not the key principle. The key is the differential expansion of the two metals.

Answer:

The two metals expand differently.