A scientist performs an experiment with a sealed source that emits $\beta$-particles. The distance travelled by the $\beta$-particles in air is around $30\,\text{cm}$. Which precaution would be the most effective for protecting the scientist from the radiation?
- Ahandling the source with long tongs
- Bkeeping the temperature of the source low
- Copening all windows in the laboratory
- Dwashing his hands before leaving the laboratory