A radioactive source is positioned $2.0\text{ cm}$ from a Geiger-Müller (G.M.) tube linked to a counter. A sheet of paper is inserted between the source and the G.M. tube. The counter reading falls, but it remains above the background radiation count rate. The paper is then replaced with a sheet of aluminium, and the count rate falls to the background radiation count rate. What radiation is emitted by the source?
- Aalpha-particles ($\alpha$-particles), beta-particles ($\beta$-particles) and gamma radiation ($\gamma$-radiation)
- Balpha-particles ($\alpha$-particles) and beta-particles ($\beta$-particles) only
- Calpha-particles ($\alpha$-particles) and gamma radiation ($\gamma$-radiation) only
- Dbeta-particles ($\beta$-particles) and gamma radiation ($\gamma$-radiation) only