A scientist is performing an experiment with a sealed source that gives off $\beta$-particles. The distance travelled by the $\beta$-particles through air is approximately 30 cm. Which precaution would give the scientist the greatest protection 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