Many species of plants have nodes that are filled with nitrogen-fixing bacteria on their roots. The nitrogen that the bacteria "fix" is then used by the plant to build plant tissues. Where does most of the nitrogen that the bacteria use come from?

i believe it would be the lungs.

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.