What is meant by the half-life of a radioactive isotope?
a) The time required for the activity of a radioactive sample to fall to 1/e of its initial value.
b) The time required for the mass of a radioactive sample to decrease by one-half as nuclei decay.
c) The time required for half of the nuclei in a sample to decay.
d) The time required for the activity of a radioactive sample to double.