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。
population 與 popularity 開頭幾個字母相同，掃過去容易當成同一個字。兩者其實是兩個不同的字：population 是「人口、族群」，popularity 是「受歡迎的程度」（gain popularity 大受歡迎）。
先確認空格在句子裡的身分。that 子句的主詞是 the ___ of teenagers，動詞是句尾的 is，中間的 it can affect 是省略了關係代名詞的子句，修飾前面那個名詞——完整說法是 the ___ of teenagers which it can affect。看出這一層很重要：空格是主詞的核心名詞，不是動詞，也不是修飾語。接著看它要配什麼。述部是 is very wide（範圍非常廣），「廣」形容的是範圍的大小；能被說成寬廣的是一個群體的涵蓋面，不是一個程度值。受歡迎的程度只能說高或低（high／low popularity），不會說寬或窄，搭配上就對不起來。驗算在下一句：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。）講的是「能接觸到的人很多」，正好呼應族群範圍廣。順帶記一個用法：population 不只指一國人口，也能指某個特定群體——the elderly population（高齡族群）、the student population（學生族群）、the local population（當地居民）。
四個都是名詞，句法上都放得進去，錯在語意與搭配。popularity 是程度，配的是 high／low／growing，不配 wide。pollution 是「污染」，與青少年、與 affect 的對象都接不上，只是字首相近。possibility 是「可能性」，說 the possibility of teenagers 語意不成立——可能性後面接的是事件（the possibility of rain），不是一群人。
the population of（…的族群、人口）、the elderly／student population（高齡／學生族群）、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.