The man then took the worm out of the first jar and put it in the second jar 18 contained alcohol and said to the audience, “Now watch the reaction.”
答案：(C) which
which 引導形容詞子句修飾先行詞 jar，並在子句中作主詞（那個罐子裝著酒精）。what 不能有先行詞、when／where 分別修飾時間／地點先行詞，皆不符合這裡修飾「罐子」這個物品先行詞的語法。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
jar 是個罐子，罐子就是容器，容器就是「地方」，所以 where 讀起來特別順——「放進第二個罐子，那裡面裝著酒精」。中文這樣講完全通，英文卻不行，差別在空格後面那個子句缺不缺東西。
關係詞這一類題有一個自己動手就能驗算的方法：先找出前面那個名詞（先行詞），再把它整個搬回空格後面的子句裡，看句子成不成立、它站的是哪一個位置。本題的先行詞是 the second jar，搬回去就成了 The second jar contained alcohol.（第二個罐子裝著酒精。）句子完全成立，而且它站的是主詞的位置——主詞被抽走了，空格就要填一個代替主詞的關係代名詞，答案是 which。where 之所以不行，正是因為它是關係副詞：關係副詞後面的子句必須自己完整，像 the jar where he put the worm（他把蟲放進去的那個罐子），子句裡 he 是主詞、the worm 是受詞，成分都齊了；本題的 contained alcohol 少了主詞，非得由關係代名詞補上不可。what 也出局，理由在劍橋 English Grammar Today 的 what 條目：what 自己就帶著先行詞（等於 the thing that），前面若已經站著一個名詞，就得改用 which 或 that，所以英文說 This is the book which the lecturer mentioned.，不能說成 the book what ...。
when 的先行詞必須是時間（the day when we met），jar 不是時間；where 與 what 的問題如上。這三個選項都不是敗在語意，而是敗在子句的成分上——它們都沒辦法在 contained 前面補出一個主詞。
which 引導的關係子句（補充說明前面的名詞，並代替子句裡的主詞或受詞）、關係副詞 where（先行詞是地方，且後面的子句必須完整）、take something out of（把某物從……取出）、put something in（把某物放進……）、say to somebody（對某人說）、reaction（反應）
A man wanted to prove to a group of alcoholics the effect of alcohol on human body systems. He displayed two jars in front of the group, one containing water and the other containing alcohol, 16 a very healthy worm. He said to the audience: "This jar contains water." Then he dropped the worm in the jar and said, "Please watch the reaction." The group watched closely 17 the worm floated in the jar, dangling and swimming.
The man then took the worm out of the first jar and put it in the second jar 18 contained alcohol and said to the audience, "Now watch the reaction." The group saw the worm go down into the water and soon begin to struggle for 19. Right there and then every one in the group witnessed the worm shrinking and disintegrating. Then it died.
The man turned round and asked the audience, "What would you all say to this?" After a long silence, someone from the rear 20 and said, "I can see that if I drink alcohol, there will be no more worms in my body."