Why E?

This is a case where the test makers may have been a little liberal with their word choice. Two potential hypotheses were mentioned, which is literally "a number," although not the traditional use of the term. In this case, it's best to look at the rest of the answer, which will prove that E is correct. E lays out the paragraph as: - Introduce hypotheses - Favorable comments - Advocates for more favorable answer Paragraph two is: - Mention two hypotheses as potential examples - Says they may help explain, but there is another option - Explain for a few sentences some background - Compare that to the author's work, say this helps explain better That shows that E is a very good layout of paragraph 2.

I was also confused about "a number" in answer choice E. Thank you for the explanation, but then why is answer choice B wrong then?

@Samantha-Alexis, (B) says, "The author shows that two explanatory hypotheses are incompatible with each other and gives reasons for preferring one of them." There is no suggestion in the second paragraph of the passage that the hypotheses mentioned are incompatible with each other, so we can get rid of this choice. Does this make sense? Let us know if you have any other questions!