Critical Reasoning

PLease help me find out a approach such kind of question?

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 entered their fi eld at an older age
than is usual. Since by the age of forty the large
majority of scientists have been working in their fi eld
for at least fi fteen years, the studies' fi nding strongly
suggests that the real reason why scientists over forty
rarely produce highly creative work is not that they
have aged but rather that scientists over forty have
generally spent too long in their field.

In the argument given, the two portions in boldface
play which of the following roles?
(A) The fi rst 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 fi rst is an objection that has been raised
against a position defended in the argument;
the second is that position.
(C) The fi rst is evidence that has been used to
support an explanation that the argument
challenges; the second is that explanation.
(D) The fi rst 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 fi rst provides evidence to support an
explanation that the argument favors; the
second is that explanation.