Four identical light bulbs are arranged in a circuit as shown. What is the minimum number of switches that must be closed for at least one light bulb to come on?

1. 1
2. 2
3. 3
4. 4
5. 0

Two switches is the minimum. Switch D for certain, and either switch B or switch C.

Which of the switches should be closed to maximize the brightness of bulb D?

1. All four switches.
2. Switch D and either switch B or switch C
3. Switch D and both switches B and C
4. Switch A, either switch B or switch C, and switch D
5. Only switch D.

All four switches should be closed to make bulb D as bright as possible. The more current a bulb gets the brighter it is, and the current in the circuit is maximized when the overall resistance of the circuit is minimized. Closing switch A takes bulb A out of the circuit, and closing switches B and C puts those bulbs in parallel, reducing the resistance. Switch D must be closed to complete the circuit.