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It shows that there are more sources for the reader to read, and it also is strong supporting evidence for the main idea.
The answer is:
B. It is in iambic pentameter and doesn't rhyme.
That is why it's called blank verse! :)
In this poem, Pound is comparing "faces in the crowd" to petals that are laying against a tree branch.
This is exactly what he says in the poem - "petals on a wet, black bough." A bough is the main branch of a tree, so this is definitely the only appropriate answer in this case. He wants to say that people resemble these petals.