Answer is: C. radioactive isotopes last for less time.
For example, polonium is rare and highly radioactive metal with no stable isotopes. Due to the short half-life of all its isotopes, its natural occurrence is limited to tiny traces of the fleeting polonium-210 (with a half-life of 138 days) in uranium ores.
On other hand, the half-life for the radioactive decay of U-238 (stable isotope) is 4.5 billion years and is independent of initial concentration.