Priya built the circuit as shown in the figure given below. If she wants both bulbs (B1 and B2) to glow, which switches (S1, S2, S3) should be kept ON?

S1 and S2 only

Reason — The bulb B1 lies in the main circuit, so it can glow only when the circuit is complete. This requires the switch S1 (which lies in the main line) to be ON.

The bulb B2 together with the switch S2 forms one branch, and the switch S3 forms a second branch connected in parallel with it. If S3 is put ON, it provides a path of practically zero resistance across the branch containing B2, so B2 gets short circuited and no current passes through it.

Hence, both the bulbs glow only when S1 and S2 are ON and S3 is OFF.