A teacher places a radioactive source close to a detector. The detector gives a reading of $320\text{ counts/min}$. After the source has been taken away, the detector is switched on again and its reading is $20\text{ counts/min}$. What is the counts/min caused only by the source, and why does the detector still show a reading when no radioactive source is present?
- A$300\text{ counts/min}$, zero error on detector
- B$300\text{ counts/min}$, background radiation
- C$340\text{ counts/min}$, zero error on detector
- D$340\text{ counts/min}$, background radiation