You are presented with the quiz "What do geologists use to determine the age of rock layers?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "radioactive dating is used to determine the age of fossils" and "geologists use fossils to determine the age of the rock layer that it was found in". 

So, what's the best answer to the question?


 

       Radioactive dating    