Question 20
Switch A turns on/off lights 1 and 2. Switch B turns on/off lights 2 and 4. Switch C turns on/off lights 1 and 3. Given that: an open circle means ON and a filled circle means OFF. At first, 4 lights are presented as Figure 1. Then, Peter pressed switch C, switch A and switch B in that order. He noticed the lights changed as in Figure 2. Which switch(es) do(es) not work?
