Why Is the Inside of the Earth Still Hot

Multiple Choice

Why is the inside of the Earth still hot?

A) Because the outer layers have prevented the escape of heat trapped during the planet's initial formation, and no new heat has been formed.
B) Because the decay of large radioactive elements is creating heat in the Earth's inner layers.
C) Because a nuclear process like that found in stars is at work in Earth's interior.
D) Because huge quantities of oil and natural gas occasionally burn deep within the Earth.