The graph illustrates how the count rate recorded by a counter placed close to a sample of a radioactive isotope varies during a period of several days. The background count rate is $5$ counts per minute. What is the half-life of the isotope?
- A$2.0\ \text{days}$
- B$2.5\ \text{days}$
- C$3.0\ \text{days}$
- D$4.0\ \text{days}$