An experiment is performed to measure radiation emitted by a radioactive source with a half-life of 10 minutes. The source is positioned near a detector which is linked to a counter, as shown. The mean background count-rate is 20 counts/minute. At the beginning of the experiment, the count-rate shown by the counter is 1000 counts/minute. What is the count-rate 10 minutes later?
- A490 counts/minute
- B500 counts/minute
- C510 counts/minute
- D530 counts/minute