Natural selection is often described with the phrase "survival of the fittest." From the examples in this tutorial

and the variations you see in organisms around you, why do you think the phrase uses "fittest" rather than

strongest, brightest, or largest?

Because fittest is not used to mean strongest, brightest, or largest. Fittest means most or best adapted. So organisms who are most or best adapted survive. Hope this helps!

"d.) carbon dioxide and hydrogen sulfide"

blue stars have the highest surface temp

stellar distances are usually expressed in light-years

the measure of a star's brightness is called magnitude

hope this !