An experiment is carried out to determine the radiation from a radioactive source. The source has a half-life of $10$ minutes. The source is positioned near a detector linked to a counter, as shown. The mean background count rate is $20$ counts/minute. At the beginning of the experiment, the counter shows a count rate of $1000$ counts/minute. What is the count rate $10$ minutes later?
- A$490$ counts/minute
- B$500$ counts/minute
- C$510$ counts/minute
- D$530$ counts/minute