Why is natural gas referred to as a fossil fuel?

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

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

CNatural 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.

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