A radioactive material gives out radiation at a rate of $600$ emissions per second. After four hours, its radiation rate is $300$ emissions per second. Determine the substance’s half-life, and state the emission rate after another four hours.
- Ahalf-life $2$ hours; $0$ emissions per second
- Bhalf-life $2$ hours; $150$ emissions per second
- Chalf-life $4$ hours; $0$ emissions per second
- Dhalf-life $4$ hours; $150$ emissions per second