D, for example(disregarding losses Chris mentioned), if you have one speaker using 1 watt for a relatively loud 85dB at the listening position and then add a second speaker also using 1 watt for 85dB, 2 watts are now used, but the combined output is now 3dB higher, for a new 88dB sound level. If the overall sound level was instead to remain at the original 85dB, each speaker would use 1/2 watt for an 82dB output, with the result that 1 watt would be used for an 85dB sound level, as was the case using a single speaker.
