Uranium - Lead Rubidium - Strontium Potassium - Argon Uranium - Strontium
I gave my students this multiple choice question: Geologists frequently use mass spectrometers in determining the relative age of rocks. A spectrometer can differentiate between the ratios of isotopes in rock samples and calculates the radioactive decay involving what two isotopes?

Only one answer is correct among these 4 choices:
- Uranium - Lead
- Rubidium - Strontium
- Potassium - Argon
- Uranium - Strontium

Could you tell me which one is correct?

Uranium - Lead