How Are Radioactive Materials Used to Determine the Age of an Object

How are radioactive materials used to determine the age of an object?

A) Both the amounts of radioactive and decay material are measured. Using these with the radioactive material's half-life, the age can be estimated.
B) The amount of radioactive material is measured. Using this with the radioactive material's half-life, the age can be estimated.
C) The amount of decay material is measured. Using this with the radioactive material's half-life, the age can be estimated.
D) The amount of heat generated by radioactive dating is measured to determine age.