To establish the age of a rock or a fossil, researchers use some type of clock to determine the date it was formed. Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events.
A is the answer, this is because deposition has been happening on earth for ages so as time goes by more deposits form layers on earth, including the remains of animals and plants that would later become<u> </u><u>fossils.</u>
