能解释一下, 答案为什么是D？

Q20:

Scientists typically do their most creative work before the age of forty. It is commonly thought that this happens because aging by itself brings about a loss of creative capacity. However, studies show that a disproportionately large number of the scientists who produce highly creative work beyond the age of forty entered their field at an older age than is usual. Since by the age of forty the large majority of scientists have been working in their field for at least fifteen years, the studies’ finding strongly suggests that the real reason why scientists over forty rarely produce highly creative work is not that they have simply aged but rather that they generally have spent too long in a given field.

In the argument given, the two portions in boldface play which of the following roles?

The first is the position that the argument as a whole opposes; the second is an objection that has been raised against a position defended in the argument.
The first is a claim that has been advanced in support of a position that the argument opposes; the second is a finding that has been used in support of that position.
The first is an explanation that the argument challenges; the second is a finding that has been used in support of that explanation.
The first is an explanation that the argument challenges; the second is a finding on which that challenge is based.
E.The first is an explanation that the argument defends; the second is a finding that has been used to challenge that explanation.

first of all, you post this up in the wrong section, but for the hell of it:

"aging by itself brings about a loss of creative capacity", the conclusion of the argument should be "the studies’ finding strongly suggests that the real reason why scientists over forty rarely produce highly creative work is not that they have simply aged but rather that they generally have spent too long in a given field.", in short "artists are not being less creative because they are over forty BUT BECAUSE...", so from this you know the author is arguing aginst the point of "aging by itself brings about a loss of creative capacity", so the answers are either D or C;

now think about how does an author challenge this argument; he challenges this argument by pointing out a new thoery based on a new finding, "a disproportionately large number of the scientists who produce highly creative work beyond the age of forty entered their field at an older age than is usual", so this is what the author is using to support his argument, thus the answer is D.

many thanks !!!