How many regions make up the brain stem?

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.

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.