Question 10
An electrical circuit wiring four lights depends on switches A, B, C and D. A toggles lights 1 and 2, B toggles lights 2 and 4, C toggles lights 1 and 3, D toggles lights 3 and 4 (on becomes off and off becomes on). Switches D, A, B and C are thrown in turn and Figure 1 becomes Figure 2. One switch is not working. Which one?
