Tritium Is a Radioisotope of Hydrogen Having a Half-Life of 12.3

Tritium is a radioisotope of hydrogen having a half-life of 12.3 years. If you initially had 1.0 * 10^-7 mole of tritium, how many moles of tritium would be left after 78 years

A) 8.0 * 10^-10 mol
B) 1.0 * 10^-9 mol
C) 1.2 * 10^-9 mol
D) 1.4 * 10^-9 mol
E) None of the above