A student studies four radioactive isotopes. A detector is positioned close to each radioactive sample. The background count rate remains at 36 counts per minute for the whole investigation. The table gives the detector readings at the beginning and after 8 hours. Which isotope has a half-life of 4 hours?
- Acount rate at the start: $150$ counts per minute; count rate after $8$ hours: $36$ counts per minute
- Bcount rate at the start: $212$ counts per minute; count rate after $8$ hours: $53$ counts per minute
- Ccount rate at the start: $260$ counts per minute; count rate after $8$ hours: $92$ counts per minute
- Dcount rate at the start: $356$ counts per minute; count rate after $8$ hours: $80$ counts per minute