Why is oil referred to as a fossil fuel? a. oil was formed from decaying layers of plants in swamps millions of years ago. they fossilized into oil. b. oil formed from dead marine organisms that fell to the ocean floor, were covered by ocean sediments, and broken down by bacteria into oil over millions of years. c. oil was formed from the breakdown of igneous rocks over millions of years. d. oil was formed from black shales that were uplifted millions of years ago and fossilized into oil.