一道逻辑题求解[求助]

In a study of the effect of color on productivity,50 of 100 factory workers were moved from their drab workroom to a brightly colored workroom. Bothe these workers and the 50 who remained in the drab workroom increased their productivity, probalbly as a result of the interest taken by researchers in the work of both groups during the study.Which of the following,if true, woiuld cast most doubt upon the author's interpretation of the study results given above?
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.
B.The drab workroom was designed to provide adequate space for at most 65 workers.
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 workers who remained in the drab work-room.
D. Nearly all the workers in both groups had volunteered to move to the brightly colored workroom.
E Many of the workers who moved to the brightly colored workroom reported that they liked the drab workroom as well as or better than they liked the brightly colored workroom

答案是B。搞不懂，求大仙指点。