SUGGESTED ANSWER TO QUESTION 2A-3(d)

In selecting rats for the experimental group, it seems very likely that the researcher selected those rats that already were highly active. That is, the rats that ran into his hands probably were ones that could be expected to run through the maze more quickly, even without the training. The rats that stayed still and hung back near the rear of the cage were more likely to be included in the control group. These less-active rats could be expected to run through the maze more slowly, regardless of whether they were trained or not.