In the trachea there is a layer of epithelial cells called the respiratory mucosa, which contains
specialized cells that produce mucus. These
specialized cells are called goblet cells and the
function of the mucus is to keep the trachea
moist. What is this an example of?

mountains can affect the climate of nearby lands. in some areas, mountains block rain, so that one side of a mountain range may be rainy and the other side may be a desert. much of airborne moisture falls as rain on the windward side of mountains.

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.

not: both offer magnification to the atomic level.