How can a half-life be used to tell the age of a sample?

      
1. The half-life of an isotope is constant.
      
2. The amount of radioactive form depends on the amount of radiation in the surroundings.
      
3. The ratio between the radioactive form and the stable form depends on the archaeological record.
      
4. The ratio between the radioactive form and stable form varies regularly with time.
      
5. both The ratio between the radioactive form and the stable form depends on the archaeological record and The ratio between the radioactive form and stable form varies regularly with time