Hi, this passage describes a pretty terrible experimental design. Twenty volunteers of the same sign were selected to take a personality test, and that test showed that those twenty people were more outgoing than average, which the investigators also thought of the volunteers.
There are a couple of issues here. First, the sample size is minuscule. Second, the volunteers were selected by the investigators, who may have been biased and intentionally selected more outgoing people. Third, the lack of random selection means that more outgoing people would be more likely to opt in to the study in the first place.
E is a good match for the failure to select randomly. If someone who is willing to participate were already more likely than the general population to be outgoing, then of course the results would show the sample was more outgoing than average regardless of sign.
C doesn't really tell us anything. This just tells us that the investigators's initial impressions tended to be correct, which we knew from the passage since the test was already independent confirmation.
