In Order to Employ the Radioactive Age-Dating Technique on a Sample

In order to employ the radioactive age-dating technique on a sample of rock, it is necessary to know all of these except one. Which is the exception?

A) the amount of the radioactive element in the rock
B) the amount in the rock of the stable element into which the radioactive element decays
C) the amount in the rock of some nonradiogenic element for reference
D) the mass of the rock when it was formed