What would happen to the cycling of nutrients, including water, oxygen and carbon dioxide, if plants died out on Earth?

Water would become less used and more abundant, oxygen would be less produced, and carbon dioxide would become more abundant because the plants won't take it in to produce glucose.

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.