A detector of ionising radiation records a background count rate of $20$ counts/minute. A radioactive isotope whose half-life is $2.0$ days is placed near the detector. The detector reading rises to $100$ counts/minute. How long is it before the detector reading falls to $40$ counts/minute?
- A$2.0$ days
- B$4.0$ days
- C$5.0$ days
- D$10$ days