Your answer would be A
Homologous chromosomes trade pieces of DNA.
When crossing over occurs, segments of non-sister chromatids break and then reattach to the other chromatid.
That evolution accord as dramatic/ rapid changes after periods of no change. Meaning their was no change over the years of humans fossils and living humans
It helps sustain tbe population because of the food chain. everything needs to eat, so the more diverse a population is, the more options things have to eat. The population is sustained because everything is able to eat to survive