Older rock layers contain fossils of many kinds of organisms that are not in younger rock layers. What most likely happened to the kinds of organisms that became fossilized? A. They became extinct, so they did not exist when the younger rock layers formed. B. They formed fossils in the younger rock layers, but those rock layers broke down. C. They evolved into new organisms that could not become fossilized. D. They stopped becoming fossilized because their environment changed​