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What caused the initial sea otter collapse?

Collapse of seals, sea lions & sea otters in North Pacific triggered by overfishing of great whales. ... Lead author Alan Springer (University of Alaska, Fairbanks) and his co-authors propose that the decimation of great whale (baleen and sperm) populations by overfishing removed a major source of food for killer whales.

Explanation:

also the answer with the link was deleted

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.