Electric circuits — practice question

The diagram shows a circuit with four switches, $S_1$, $S_2$, $S_3$ and $S_4$. Which three switches have to be closed for the heater to operate?

• A$S_1$, $S_2$ and $S_3$
• B$S_1$, $S_2$ and $S_4$
• C$S_1$, $S_3$ and $S_4$
• D$S_2$, $S_3$ and $S_4$