Fact 1: Radioactive dating is used to determine the age of fossils. Fact 2: Geologists use fossils to determine the age of the rock layer that it was found in. Given the two facts above, what do geologists use to determine the age of rock layers?

OPT:
1). Chemical dating.
2). Mechanical dating.
3). Non radioactive dating.
4). blastoids.
5). single-celled organisms.
6). Seismometer.
7). Radioactive dating.
8). an electron microscope.
The answer is:
7).