gmat一道逻辑题

gmat一道逻辑题
An overwhelming proportion of the most productive employees at SaleCo’s regional offices work not eight hours a day, five days a week, as do other SaleCo employees, but rather ten hours a day, four days a week, with Friday off.Noting this phenomenon, SaleCo’s president plans to increase overall productivity by keeping the offices closed on Fridays and having all employees work the same schedule—ten hours a day, four days a week.
Which of the following, if true, provides the most reason to doubt that the president’s plan, if implemented, will achieve its stated purpose?
A. Typically, a SaleCo employee’s least productive hours in the workplace are the early afternoon hours.
B. None of the employees who work four days a week had volunteered to work that schedule, but all were assigned to it by their supervisors.
C. Working ten hours a day has allowed the most productive employees to work two hours alone each day in their respective offices relatively undisturbed by fellow employees.
D. Employees at SaleCo are compensated not on the basis of how many hours a week they work but on the basis of how productive they are during the hours they are at work.
E. Those SaleCo employees who have a four-day workweek do not take any of their office work to do at home on Fridays.
答案是c,不懂...请教一下大家