A student uses a detector of ionising radiation to measure the count rate close to a radioactive source. The arrangement is shown in the diagram. The counter shows $75$ counts per minute. After the source is removed, the counter reading falls to $5$ counts per minute. What is the rate of emission from the radioactive source when the counter reading is adjusted for background radiation?
- A5 counts per minute
- B15 counts per minute
- C70 counts per minute
- D80 counts per minute