In the diagram, there are three $20\,\Omega$ resistors, $R_1$, $R_2$ and $R_3$, together with a switch $S$, all connected to a d.c. power supply. Switch $S$ is open. The potential difference (p.d.) across $R_3$ is $9.0\,\text{V}$. What is the p.d. across $R_3$ when switch $S$ is closed?
- A$3.0\,\text{V}$
- B$6.0\,\text{V}$
- C$12\,\text{V}$
- D$27\,\text{V}$