A tracer must have an activity of at least 80 MBq80\ \mathrm{MBq}80 MBq at the end of a 4-hour investigation. To limit later exposure, its activity must be no more than 10 MBq10\ \mathrm{MBq}10 MBq after 16 hours. It remains in the system, and its activity changes only by radioactive decay. Which source meets both conditions?
| Source | Initial activity in MBq | Half-life in hours |
|---|---|---|
| P | 160 | 2 |
| Q | 160 | 4 |
| R | 160 | 8 |
| S | 320 | 4 |
Source R
Source P
Source S
Source Q