Gateway drugs are substances people take that 21 them to take more drugs.
答案：(A) lead
關係子句的主詞是先行詞 substances（複數），動詞須用原形 lead；leads 是單數形，leading 與 led 都不能當關係子句的動詞。
答對率 36%。
空格前面最近的名詞是 people（複數）也好、往前一點是 substances（複數）也好，你可能被中間的 people take 干擾，以為主詞是別的字，於是挑了 leads。
完整結構是 Gateway drugs are substances (that people take) that ___ them to take more drugs.——第二個 that 的先行詞是 substances（那些物質），所以動詞要配合複數，用原形 lead。把中間的 people take 用括號括起來，句子的骨幹就清楚了。
lead sb to V ＝ 促使某人去做某事：What led you to change your mind?（是什麼促使你改變心意？）本句是「這些物質促使人去用更多毒品」——主動去促使的是「物質」，不是「人」。
leading 是現在分詞、led 是過去式或過去分詞，兩者都不能單獨擔任關係子句裡的限定動詞。
lead sb to V（促使某人做…）、lead to（導致）、a gateway drug（入門毒品）、關係子句的動詞須配合先行詞。
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.