The diagram represents a circuit containing a lamp, an electric bell and three switches $S_1$, $S_2$ and $S_3$. Neither the lamp nor the bell has a fault. The bell is sounding, but the lamp remains unlit. Which switches are shut?
- A$S_1$ only
- B$S_1$ and $S_2$ only
- C$S_1$ and $S_3$ only
- D$S_1$, $S_2$ and $S_3$