The Mexican horn shark has
fins and spines b/c it feeds on...?

Explanation:

They feed on benthic invertebrates, especially sea urchins, crabs and probably abalone, also fishes.

The Mexican Horn shark usually feeds on crabs and demersal fishes including midshipman. Conservation: The Mexican Horn Shark is not of commercial value but is taken as bycatch in bottom gillnets and shrimp trawling operations in the Gulf of California and Mexican coastal lagoons.

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.

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