The emission rates of four radioactive sources are recorded at intervals of $20$ minutes. The table gives the readings for each radioactive source in a separate row. Which source has the greatest half-life?
- Arate of emission (time 0): 120, (time 20 min): 60, (time 40 min): 30
- Brate of emission (time 0): 120, (time 20 min): 110, (time 40 min): 101
- Crate of emission (time 0): 240, (time 20 min): 60, (time 40 min): 15
- Drate of emission (time 0): 240, (time 20 min): 170, (time 40 min): 122