Even though ground-level ozone is a health hazard, the ozone layer in the stratosphere is a health benefit because

The ozone layer absorbs a large portion of incoming UV radiation, which keeps ground-level ozone from forming. A higher percentage of UV radiation entering the troposphere would allow a greater amount of ozone to form in the troposphere. Ozone is a pollutant in the troposphere. By regulating the amount of UV radiation that enters the troposphere, the ozone layer also regulates the amount of ozone formation in the troposphere.

Explanation:

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.

your spelling is a little off : ]