Sentence Correction

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?

A. 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.
B. 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.
C. The first is an explanation that the argument challenges; the second is a finding
that has been used in support of that explanation.
D. 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.