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.
11. Which of the following, if true, would provide most support for concluding from the survey results described above that the use of illegal drugs by people below the age of 20 is declining?
(A) Changes in the level of drug use by high school seniors are seldom matched by changes in the level of drug use by other people below the age of 20.
(B) In the past, high school seniors were consistently the population group most likely to use illegal drugs and most likely to use them heavily.
(C) The percentage of high school seniors who use illegal drugs is consistently very similar to the percentage of all people below the age of 20 who use illegal drugs.
(D) The decline revealed by the surveys is the result of drug education programs specifically targeted at those below the age of 20.（C）
(E) The number of those surveyed who admit to having sold illegal drugs has declined even faster than has the number who have used drugs.
请问为什么选C啊？怎么就支持了结论了呢？ | 
