Which one of the following, if true, would most seriously undermine the validity of the results of the experiment discussed in the last paragraph? A. The live birds all came from different titmouse flocks. B. The physical characteristics of the stuffed dummies varied in ways other than just breast-stripe width. C. No live juvenile birds were included in the experiment. D. The food placed in the feeding tray was not the kind of food normally eaten by titmice in the wild. E. Even the live birds that acted aggressively did not actually physically attack the stuffed dummies.