The deadliest of all mass extinctions, which killed off 95% of all living organisms, was the Cretaceous-Tertiary
End Triassic
Permian-Triassic
Late Devonian
Ordovician-Silurian

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.

c is the correct answer hope this !