Critical Reasoning

Musical composers have generally made their most significant contributions to the musical canon before they reached the age of thirty. It is commonly believed that this is the case because aging brings about a loss of cognitive ability and creative capacity. However, a study pointed out that a disproportionately large number of those composers who made their most significant contributions to the canon after the age of thirty became musicians at an older age than is generally the case. Since by the age of thirty many composers have been engaged as musicians for a decade or more, these findings suggest that the real reason why musicians over thirty rarely make significant contributions to the musical canon is not that they have aged but rather that they have spent too much time as musicians.

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

A)The first is an explanation that is challenged by the argument; the second is a finding used to support that explanation.
B)The first is an explanation that the argument opposes; the second is an objection raised against the alternative explanation advocated by the argument.
C)The first is a claim advanced in support of a position; the second is a finding that clarifies that position.
D)The first is an explanation advocated by the argument; the second is a finding used to challenge that explanation.
E)The first is an explanation that the argument challenges; the second is a finding on which that challenge is based.

OA is E but can someone explain me how?

The first boldface portion is the explanation that is "commonly believed", but the argument raises a challenge to that explanation. That narrows it down to A or E. The second boldface portion is evidence that does not support the conventional explanation, but rather it supports the argument's challenge. Thus the answer is E.