When the two solutions are mixed , a clear solution with white powder at the bottom results.,What has caused this change?

Chemical reaction

Explanation:

Chemical changes happens when a substance combines with another to form a new substance (or unexpected color, temperature or smell), called chemical synthesis or, alternatively, chemical decomposition into two or more different substances

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.

regents of the university of california v. bakke, 438 u.s. 265 (1978), was a landmark decision by the supreme court of the united states. it upheld affirmative action, allowing race to be one of several factors in college admission policy.