Another reason why alcohol is the main gateway drug is that the 25 of teenagers it can affect is very wide.
答案：(A) population
population 在此指「（青少年）族群、人口」：酒精能影響的青少年族群範圍非常廣；popularity（受歡迎程度）、pollution（污染）、possibility（可能性）語意都不合。
答對率 33%。
這題是典型的形似字陷阱——population 與 popularity 只差幾個字母，趕時間時很容易看錯。population ＝ 人口、族群；popularity ＝ 受歡迎的程度（gain popularity 大受歡迎）。
句尾 is very wide（範圍非常廣）——「廣」形容的是族群範圍，不是受歡迎程度；下一句 It is easily accessible in most societies and common in popular events such as sports gatherings（酒精在多數社會都容易取得，在運動聚會等場合也很常見）進一步說明「能觸及的人很多」。
population 不只指一國人口，也可以指某個特定群體：the elderly population（高齡族群）、the student population（學子群體）。
the population of（…的族群、人口）、gain popularity（大受歡迎）、be accessible to（容易取得）、a gateway drug（入門毒品）。
Recent studies have shown that alcohol is the leading gateway drug for teenagers. Gateway drugs are substances people take that 21 them to take more drugs. Alcohol works directly on the central nervous system and alters one's moods and limits judgment.
Since its way of altering moods (changing one's state of mind) is generally expected and socially acceptable, oftentimes it 22 over drinking. Habitual drinkers may find alcohol not stimulating enough 23 and want to seek other more stimulating substances. 24 a circumstance often preconditions teenagers to the possibility of taking other drugs such as marijuana, cocaine or heroin.
Another reason why alcohol is the main gateway drug is that the 25 of teenagers it can affect is very wide. It is easily accessible in most societies and common in popular events such as sports gatherings and dinner parties.