这题怎么理解？ 为什么选C我选的D

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?

1. Typically, a SaleCo employee’s least productive hours in the workplace are the early afternoon hours.
2. 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.
3. 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.
4. 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.
5. Those SaleCo employees who have a four-day workweek do not take any of their office work to do at home on Fridays.

请各位大侠指点迷津谢谢！

我觉得是否直接从“他因解释”理解更方便？

原文的前提在于它提出的现象：10小时4天的人比8小时5天的人效率高。

也就是说原文认为是工作时间的安排是效率高的唯一原因

那驳斥选项就是说是因为别的原因影响了效率。C说明的正是其他原因