During implantation, the trophoblast erodes a path through the endometrium. maternal blood vessels in the endometrium are walled off from the blastocyst. inner cell mass begins to form the placenta. entire trophoblast becomes syncytial inner cell mass is temporarily deprived of nutrients

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.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.

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