Both coral reefs and wetlands have been destroyed by

human activity

Explanation:

The wetlands and the coral reefs have gradually been destroyed by the humans and their activity. The wetlands were the first to be under the human influence and getting destroyed, while the coral reefs have come to have similar destiny in the last century. The reasons for the destruction are different. The wetlands have in general been destroyed on purpose, be it because they were epicenter of diseases, for making more land for agriculture, or making more land for settlements. The coral reefs have not been destroyed on purpose, but the human activity has resulted in lot of pollution of the ocean waters, as well as changes in the climate, and the fragile corals can not adapt quickly enough to these changes, so they die out quickly.

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.