Uranium- `238` decays to thorium-`234` with half-life `5xx10^(9) yr`. The resulting nucleus is in the excited state and hence further emits `gamma`-rays to come to the ground state. It emits `20 gamma`-rays per second. The emission rate will drop to `5 gamma`-rays per second in
A. `1.25xx10^(9) yr`
B. `10^(10)yr`
C. `10^(-8)yr`
D. `1.25xx10^(-9)s`