A radioactive substance gives off radiation at a rate of 600 emissions per second. After four hours, its radiation emission rate has fallen to 300 emissions per second. What is the half-life of the substance and what is the rate of emission 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