Critical Reasoning

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 of scientists who produce highly creative work beyond the age of forty, a disproportionately large number of the scientists 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 scientists over forty have generally spent too long in their given field.

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

A) the first is a claim, the accuracy of which is at issue in the argument; the second is a conclusion drawn on the basis of that claim
B)The first is an objection that has been raised against a position defended in the agrument; the second is that position
C) The first is evidence that has been used to support an explanation that the argument challenges; the second is that explanation
D) The first is evidence that has been used to support an explanation that the argument challenges; the second is a competing explanation that the argument favors
E) The first provides evidence to support an explanation that the argument favors; the second is that explanation

[spoiler]OA - E; OG 12; Pls explain meaning of a clause in C - The first is evidence that has been used to support an explanation that the argument challenges' (OG says, 'the first is not used to support a position the argument challenges) - I feel BF1 gives evidence that support the argument view that scientists drop in creativity is not the reason but time they have spent'. Pls help me understand[/spoiler]