Fact 1: Radioactive dating is used to determine the age of fossils.

Fact 2: Radioactive isotopes are commonly used for dating .

Given the two facts above, answer the question "Radioactive isotopes are commonly used to do what to the age of fossils?" with the following options: 
- duplicate itself
 - observe it
 - debate them
 - Temperature
 - Time and energy
 - determine them
 - remove them
 - assign them


 

      determine them   