What Does the Term half-Life Describe

What does the term half-life describe?

A) time needed for half of the radioactive atoms in a substance to decay into stable isotopes.
B) time needed for half of the radioactive atoms in a substance to convert into energy.
C) half the time needed for a radioactive substance to become safe.
D) time needed for half the cells in an organism to turn cancerous from radiation damage.
E) half the time needed for cells in a living organism to recover from radiation damage.