The total amount of energy in the universe never changes, but the availability of each different type of energy to do work can change tremendously. What best explains this change in available energy?
a
Energy can only be transferred as heat.
b
Energy is always converted with 100 percent efficiency.
c
Energy can only be created through nuclear reactions.
d
Energy is regularly changing from one form into another.
d
Energy is regularly changing from one form into another.