If a Rock Started with 1,000 Atoms of a Parent

Multiple Choice

If a rock started with 1,000 atoms of a parent but now contains 250 atoms,how many half-lives have passed?

A) 0.25 half-lives
B) 0.5 half-lives
C) 2 half-lives
D) 4 half-lives
E) There is no way to tell.