A certain mutation in the gene for hemoglobin results in the red blood cells becoming sticky, rigid, and irregularly shaped. These irregularly shaped red blood cells block the flow of blood
throughout the body. A single base mutation is responsible for these irregularly shaped blood
cells.

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.