The half-life of a radioactive isotope is $24\,\text{hours}$. A sample of this isotope gives an initial count rate of $720$ counts per second. How long will it take for the count rate to decrease to $90$ counts per second?
- A$3\,\text{hours}$
- B$72\,\text{hours}$
- C$96\,\text{hours}$
- D$192\,\text{hours}$