Making educated guesses (rather than randomly guessing) gives you a chance of answering the question correctly.

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.

which

for a different answer set the answer would be because the oxygen in the water had depleted (apex)