Why can't a star's distance be determined solely from its apparent brightness in the sky? (a) stars move too fast (b) stars burn out before we can determine their distance (c) stars have different energy outputs (d) there's no way to measure the light from a star (e) earth's atmosphere blocks all starlight

(C) stars have different energy outputs

Explanation:

All the stars that we see in the Universe are not of the same size and brightness. Also, they are not situated at the same distance from us. If all the stars had been of the same size and luminosity, we could have determined their distance with the help of their apparent brightness. Also there can be two stars of equal size but their apparent brightness might be different. The brightness depends on the size and the age of the stars. There can be a very bright star which looks very faint to us just because its farther in comparison to other stars.

Hence, we can not rely solely on the apparent brightness of the stars as they have different energy outputs.