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I was confused between answer choices A and E for this one.
Is A incorrect because it leaves out the part about researchers being puzzled by how kinglets are able to survive cold winter nights? I was trying to figure out if answer choice A had any descriptive/factual errors, but I couldn't find any, so I'm guessing the problem is that it doesn't address the fact that scientists are trying to figure out the mechanisms behind the birds' survival.
That seems to be the only difference between A and E.