I can not avoid to think about coal, but you said that a material is not the answer. So I can only think about a black body, but thought that was too abstract, like saying "absorber", which wasn't the answer. Let me think...

didiplaywell

science

Brain Not Braining : Part 3

noknees

superinsulators do not allow heat to flow through them along with electricity, it will be dimmer than A when heated to 1000 degrees which is contrary to the question, B appears to be a perfect emitter of heat (this is one hell of a hint)