A radioactive source has a half-life of $0.5\ \text{hours}$. A detector close to the source gives a reading of $6000$ counts per second. Background radiation may be ignored. What will the detector reading be $1.5\ \text{hours}$ later?
- A$750$ counts per second
- B$1500$ counts per second
- C$2000$ counts per second
- D$3000$ counts per second