If originally there were X atoms of radioactive isotope present and now there are only X/4 atoms, how many half-lives have elapsed?

Question

Quizplus · Accepted Answer

Half-life is the time taken for half of the atoms to decay. 
If there are X atoms initially and now there are X/4 atoms, then there have been 2 half-lives because X/2 atoms must have decayed to get to X/4 atoms. 
Therefore, Choice C is the best answer.

If Originally There Were X Atoms of Radioactive Isotope Present