Critical Reasoning

For nearly a century, physiologists erroneously believed that a buildup of lactic acid in muscle tissue was responsible for the soreness that many people experience after strenuous exercise. The acid, they claimed, is the waste produced by metabolic activity in the muscle and reaches "threshold" levels, causing soreness, when the muscle has depleted its oxygen supply. Researchers have recently discovered, however, that lactic acid is actually the fuel that powers muscular activity. Therefore, the cause of muscle soreness remains unknown.

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

A. The first is an assertion that the author accepts as true; the second is a consideration in support of that assertion.
B. The first is an assertion that the author accepts as true; the second describes a situation that the author posits as contrary to that assertion.
C. The first is an assertion that the author argues against; the second is evidence presented as contrary to the author's argument.
D. The first is evidence that the author believes is no longer valid; the second is additional evidence that the author uses to support his main point.
E. The first is a claim that the author believes to be invalid; the second is the author's main point.