How do magnetic stripes of iron in ocean rock provide evidence of seafloor spreading

They show how the density of the seafloor has changed. They reveal a matching pattern on either side of the ridge. They reveal differences in the mineral composition of newly created rock.

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.