A constant is something that is kept the same throughout the course of a experiment. So I would guess C) because they are not doing anything special. If it’s not C then it is A. :)
Im not 100% sure but the best answer I can give is Prokaryotes because they came to be around the time oxygen did as well.
Economics - is the social science that describes the factors that determine the production, distribution and consumption of goods and services
Selective selection,since being smaller is an advantage the rabbits got smaller similar to evolution