An explosion at a nuclear reactor dispersed the isotope caesium-137 over a wide region. Ninety years after the explosion, the amount of caesium-137 remaining will be $12.5\%$ of its initial level. What is the half-life of caesium-137?
- A$11.25$ years
- B$22.5$ years
- C$30.0$ years
- D$45.0$ years