(D) says, "Recent research into neurogenesis in canaries refutes a long held belief about the limited supply of brain cells and provides new information about neurogenesis in the adult human brain."
Even though the passage's author is hopeful that the recent studies of canaries show that neurogenesis is possible in vertebrate animals (look at lines 50-52), the research alone does not refute longheld beliefs around the limited supply of brain cells in vertebrate animals. If canaries are the only vertebrate animal that has the ability to generate brain cells, then the older theories are still more or less right. Additionally, the new research by itself has not provided any new information pertaining to neurogenesis in the adult human brain. Thus, (D) overstates the implications of the canary research, which allows us to get rid of this answer choice.
(A) says, "New evidence of neurogenesis in canaries challenges an established neurological theory concerning brain cells in vertebrates and suggests the possibility that human brains may repair themselves."
The author provides her conclusion in two separate parts of this passage. The first part of her conclusion is provided to us in the second paragraph, where she says that new research involving canaries provides evidence that neurogenesis exists, which undermines the old theory that vertebrates develop a fixed number of neurons early on in their lives (lines 10-12). The author then waits until the fourth paragraph to provide the other half of her conclusion, which is that the new research may help to uncover a mechanism that'll enable the human brain to repair itself through neurogenesis (lines 50-52). These two points are what the other is attempting to prove throughout the passage (that new evidence undermines an old theory about neurology and suggests that the human brain might be able to repair itself by generating neurons). Of the answer choices, (A) is the only one that includes both components of the author's conclusion, so it's the correct answer choice.
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