rare earth isotopes son isotopes daughter isotopes parent isotopes
I gave my students this multiple choice question: To calculate the relative age of rocks, geologists use the rate of radioactive decay of isotopes present in their samples. What is the term used for the original amount of the elemental isotopes being investigated?

Only one answer is correct among these 4 choices:
- rare earth isotopes
- son isotopes
- daughter isotopes
- parent isotopes

Could you tell me which one is correct?

parent isotopes