Help! please!! due in half in hour!

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

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)