In a study of the effect of color on productivity, 50 of 100 factory

In a study of the effect of color on productivity, 50 of 100 factory workers were moved from their drab workroom to brightly colored workroom. Both these workers and the 50 who remained in the drab workroom increased their productivity, probably as a result of the interest taken by the researchers in the work of both groups during the study.
Which of the following, if true, would cast most doubt upon the author's interpretation of the study results given above?

EFFECT : Both 50 workers in brightly colored workroom and the 50 who remained in the drab workroom increased their productivity
CAUSE: Probably as a result of the interest taken by the researchers in the work of both groups during the study.

For weakening such causal relations like this , check for any of the following in the answers :
1) Find an alternate cause of the stated effect
2)Show that even when the cause occurs, the effect doesn't ccur
3)Show that although the effect occurs, the cause did not occur
4)Show that the stated relationship is reversed
5)Show that a statistical problem exists with the data used to make the causal statement

A) The 50 workers moved to the brightly colored room performed precisely the same manufacturing task as the workers who remained in the drab workroom.
This answer choice points out similarity (rather than dissimilarity) between the working conditions of both the groups.Hence in one way, it stregthens that the interest of the researchers can be the cause of the increased productivity, as the working conditions were same for both the groups.
B) The drab workroom was designed to provide adequate space for at most 65 workers.
Here we find an alternate cause that might have increased the productivity of the drab room workers. The space was more there. Hence, even if the drab room workers saw the colours, the extra space made them comfortable to be more efficient in their work. Hence this answer choice puts doubt on the cause " interest taken by the researchers in the work of both groups during the study" for the increase of productivity.
This choice matches with (1) mentioned above
C) the 50 workers who moved to the brightly colored workroom were matched as closely as possible in age and level of training to the 50 workeres who remained in the drab workroom.
Similar to (A), the option points out similarity (rather than dissimilarity) between the working conditions of both the groups.Hence in one way, it stregthens that the interest of the researchers can be the cause of the increased productivity, as the working conditions were same for both the groups.
D) Nearly all the workers in both groups as volunteered to move to the brightly colored workroom.
Even though people volunteered, finally 50 were kept in the drab room. This is the fact. Hence who volunteered and who didnt, or whether people volunteered or not, is none of this argument's business
E) Many of the workers who moved to the brightly colored workroom as well as or better than they liked the brightly colored workroom.
The brightly colored room might be favorite for many, but this fact doesn't give reasons as to how the efficiency of the people in the drab room also increased.Hence in one way, this choice stregthens that the interest of the researchers can be the cause of the increased productivity, as the working conditions were same for both the groups.

Correct answer is (B)