SwitchA toggles lights 1,2; B toggles 2,4; C toggles 1,3; D toggles 3,4. C, A, D, B are operated in that order, changing Figure 1 into Figure 2. Which single switch fails completely?
A toggles lights 1,2; B toggles 2,4; C toggles 1,3; D toggles 3,4. Operate A, C, D, B in turn. One switch fails entirely. Which switch produces the illustrated change?