逻辑又一题

An annually conducted, nationwide survey shows a continuing marked decline in the use of illegal drugs by high school seniors over the last three years.

Which of the following,if true, casts most doubt on th erelevance of the survey results described above for drawing conclusions about illegal drug use in the teen-age population as a whole?

(A)Because of cuts in funding, no survey of illegal drug use by high school seniors will be conducted next year
(B)The decline uncovered in the survey has occurred despite the decreasing cost of illegal drugs
(C)Illegal drug use by teen-agers are least likely to stay in high school for their senior year.
(D)Survey articipants are more likely now than they were three years ago to describe as "heroic" people who were addicted to illegal drugs and have been able to quit

为什么答案是C? (C)只说这种情况最严重的地区,但不一定影响survey result as a whole,而D却说明这些survey participants 的特殊性,所以survey 的结果是不可靠的.