In breeding pure-breeding large and small strains of mice, you cross individuals of each strain and note that their offspring are intermediate in size. two models (explanations) to account for this result are (1) that body size in these strains is due to one gene with alleles that show incomplete dominance and (2) that body size is a polygenic trait. how could you distinguish between these models? '

backcrossing

Explanation:

when you cross the offspring (inttermediate size) to one of the parental strains, (the small strain for example)  you should get 1/2 intermediate size and 1/2 parental size (small) if body size is due to  single gene. On the other hand if the trait "size" is polygenic you should get mice around half way between small and intermediate sizes.  

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."