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?

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