A high-voltage power supply is linked to a metal grid and a wire, as illustrated. When the radioactive source is brought near the grid, sparks are seen at the position marked. Which statement explains why the sparks are produced?
- A$\alpha$-particles have a long range.
- B$\alpha$-particles have no charge.
- C$\alpha$-particles have no mass.
- D$\alpha$-particles are strongly ionising.