Here's a problem to solve: 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?

Among the 4 following options, which is the correct answer?

- A: Uranium - Lead
 
- B: Rubidium - Strontium
 
- C: Potassium - Argon
 
- D: Uranium - Strontium
 