Question 21
Switch A turns lights 1 and 2 on/off or off/on. Switch B turns lights 2 and 4 on/off or off/on. Switch C turns lights 1 and 3 on/off or off/on. Switch D turns lights 3 and 4 on/off or off/on. Switches B, D, A and C are thrown in turn, with the result that Figure 1 turns into Figure 2. Which switch does not work at all?
