A recent study of an insurance company's underwriters indicated that t

A recent study of an insurance company's underwriters indicated that those who worked in pleasant physical surroundings were 25 percent more productive than their peers in unpleasant physical surroundings. Objective criteria for evaluating job performance included caseload and complexity of cases. This shows that improving workers' environments increases those workers' productivity.

Which of the following, if true, most seriously weakens the conclusion above?

(A) On average, less-productive employees spend no fewer hours per day at their workstations than do their more-productive peers.
(B) Unpleasant surroundings give employees less motivation to work hard than more pleasant surroundings do.
(C) The more-productive employees are generally rewarded with pleasant office space.
(D) More-productive employees do not work any more hours than their less-productive peers.
(E) Peer pressure discourages employees in crowded, unpleasant surroundings from making phone calls to their own family members during work time.