How should the relationship between expansion and cooling in the Big Bang be understood?

Correct answer: Expansion spread energy through a larger volume, so temperature decreased

In the Big Bang model, the early universe was extremely hot and dense. As space expanded, the same general energy became distributed through an increasingly larger volume, and the average temperature fell. This expansion and cooling allowed particles, atoms and later larger structures to form. Hence option A gives the relevant cosmological relationship. Rivers, a static universe and the disappearance of the Sun are unrelated conclusions.