What would happen to blood pressure if the radius of a blood vessel decreases

If the radius of a blood vessel decreases, blood pressure will increase.

Explanation: A radius is the distance from the center of a circle to any part of the circumference. Blood vessels are like tubes, if the radius decreases, it means that the diameter decreases too and the vessel becomes narrower. The narrowing of the blood vessels results in increased blood pressure as the heart is being made to increase its pumping action in order to meet the body's demand for blood circulation.

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.