A nuclear reactor explosion disperses the isotope caesium-137, $^{137}_{55}\text{Cs}$, over a wide region. Once $90.0\,\text{years}$ have elapsed, the amount of caesium-137 remaining is $12.5\%$ of the initial amount. What is the half-life of caesium-137?
- A$11.3\,\text{years}$
- B$22.5\,\text{years}$
- C$30.0\,\text{years}$
- D$45.0\,\text{years}$