You walk outside on a really cold day and your face gets chilled. this is due to
a. radiation
b. convection
c. conduction
d. thermal equilibrium

a, i am pretty sure.

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.