A two-way switch has three terminals $a, b$ and $c.$ In ON position (logic value $1$), $a$ is connected to $b,$ and in OFF position, $a$ is connected to $c$. Two of these two-way switches $S1$ and $S2$ are connected to a bulb as shown below.
Which of the following expressions, if true, will always result in the lighting of the bulb ?
- $S1.\overline{S2}$
- $S1 + S2$
- $\overline {S1\oplus S2}$
- $S1 \oplus S2$