For a television program about astrology, investigators went into the street and found twenty volunteers born under the sign of Gemini who were willing to be interviewed on the program and to take a personality test. The test confirmed the investigators' personal impressions that each of the volunteers was more sociable and extroverted than people are on average. This modest investigation thus supports the claim that one' s astrological birth sign influences one' s personality. Which one of the following, if true, indicates the most serious flaw in the method used by the investigators? ['People born under astrological signs other than Gemini have been judged by astrologers to be much less sociable than those born under Gemini.', 'There is not likely to be a greater proportion of people born under the sign of Gemini on the street than in the population as a whole.', 'People who are not sociable and extroverted are not likely to agree to participate in such an investigation.', "The personal impressions the investigators first formed of other people have tended to be confirmed by the investigators' later experience of those people."]