Why does radiometric dating allow researchers to determine the age of fossils?

A)Radioactive isotopes are incorporated into living organisms easier than the corresponding non-radioactive isotope.
B)The half-life for all isotopes are all long, in the order of years.
C)The "parent" isotope decays into the "daughter" isotope at a fixed rate.
D)All radioactive isotopes have the same half-life.
E)All elements incorporated into living organisms have radioactive isotopes.