The graph indicates how the count rate recorded by a radioactivity detector positioned close to a radioactive sample varies with time. If the background count rate is $30\text{ counts/min}$, what is the half-life of this sample?
- A$3.4\text{ h}$
- B$3.6\text{ h}$
- C$4.0\text{ h}$
- D$5.5\text{ h}$