A radioactive substance emits $\alpha$-particles. The table gives the emission rate of $\alpha$-particles from the substance at various times. What is the half-life of the substance, and what is the missing value at $8.0$ hours?
- Ahalf-life $2.0$ hours; rate $0$ particles per minute
- Bhalf-life $2.0$ hours; rate $10$ particles per minute
- Chalf-life $4.0$ hours; rate $0$ particles per minute
- Dhalf-life $4.0$ hours; rate $10$ particles per minute