The total amount of energy in the universe never changes, but the amount of energy available for work at any given moment varies tremendously. What best explains this change in available energy?

Responses

Energy is always converted with 100 percent efficiency.
Energy is always converted with 100 percent efficiency.

Energy is always transferred as heat.
Energy is always transferred as heat.

Energy is continually changing from one form into another.
Energy is continually changing from one form into another.

Energy can only be created through nuclear reactions.

The correct response is: Energy is continually changing from one form into another.