Currently where are we in the cycle of biodiversity?
a. beginning of mass extinction
b. middle of mass extinction
c. near the end of mass extinction

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

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.