Which of the following correctly compares the largest sources of earth's energy?
a. the largest source of earth's internal and external energy is the sun.
b. the largest source of earth's internal and external energy is radioactive decay of heavy elements.
c. the largest internal source of earth's energy is the radioactive decay of heavy elements. the largest external source of earth's energy is the sun.
d. the largest internal source of earth's energy is the sun. the largest external source of earth's energy is the radioactive decay of heavy elements.