Why is natural gas referred to as a fossil fuel?

a. natural gas was formed from the breakdown of igneous rocks over millions of years.

b. natural gas was formed from decaying layers of plants in swamps millions of years ago. they fossilized into natural gas.

c. natural gas formed from dead marine organisms that fell to the ocean floor, were covered by ocean sediments, and broken down by bacteria into natural gas over millions of years.

d. natural gas was formed from black shales that were heated underneath by magma, thereby causing the release of natural gas.