A source contains a radioactive material. When the radioactive source is not present, a detector measures a background count rate of $20$ counts per minute. The source is then placed in a fixed position close to the detector. At first, a count rate of $520$ per minute is measured. What count rate would be measured after a time of two half-lives of the radioactive source?
- A$125$ counts per minute
- B$130$ counts per minute
- C$135$ counts per minute
- D$145$ counts per minute