What Is an agent that causes disease?

The answer is Pathogen

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.

c: warm water flows from the western tropical pacific toward south america. that should be the right answer! hope it .