Once used, the bags are sealed and stored for the flight back to the earth, 25 they are disposed of.
答案：(A) where
關係副詞 where 引導地方子句，表示「（帶回地球）在那裡被處理掉」；what（不能這樣引導子句表達地點）、which（關係代名詞，不能用來表達「在那裡」的地點語意）、whether（是否）都不合。
空格前面是逗號、後面接一串看起來完整的話，你大概會在 which 與 where 之間猶豫，而 which 是你最熟的關係代名詞，很容易第一眼就選它。再加上句尾的 disposed of 讓 of 後面看起來「少了東西」，更會覺得需要一個受詞去補，which 就更誘人。答對率四成四，多半掉在這個錯覺上。
四個選項都是 wh- 詞，先用分流判準看空格前面接的是名詞還是動詞：前面是 the earth（名詞），所以這是回頭補充說明那個名詞的關係子句，不是間接問句。接著數子句成分：they are disposed of——主詞是 they，動詞是被動的 are disposed of。這裡是最關鍵的一步：disposed of 是被動語態，of 的受詞已經被提到主詞的位置變成 they 了，所以 of 後面本來就不該再有東西，它並沒有缺。成分不缺，which 與 what 這種一定要在子句裡占一個位置的字就都進不來。剩下的 where 是關係副詞，本身不占句子成分，只代替一個表地方的介系詞片語。還原一次就清楚了：the flight back to the earth, and they are disposed of on the earth（帶回地球，然後在地球上被處理掉），把 on the earth 換成 where 就是本句（同判準：98 年第 23 題）。
which：關係代名詞，進到子句裡一定要當主詞或受詞，本句兩個位置都滿了。what：除了同樣要占位置之外，它還有一條更快的死路——what 不能當一般的關係代名詞，前面已經有名詞先行詞 the earth 時要用 which 或 that（劍橋 English Grammar Today），所以看到先行詞就能先刷掉它（同判準：93 年第 18 題、91 年第 25 題）。whether（是否）：帶出的是兩種可能性的選擇，這裡沒有任何不確定。
dispose of（處理掉、丟棄）、be disposed of（被處理掉）、get rid of（擺脫、丟掉）、seal a bag（把袋子封起來）、store sth for（把某物存放起來以備……）、once used（一經使用）。
The first American space toilet was Alan Shepard's space suit. His flight was supposed to last only 15 minutes, so there was no provision made for him to relieve himself in the capsule. His flight, 21, was delayed for hours. It was then decided that Alan Shepard should relieve himself in his space suit, and continue with the mission as scheduled.
22 problem urged NASA to develop the first space toilet, a modified diaper. Diapers are 23 used by astronauts when they are outside the space shuttle, wearing a spacesuit.
As the flights grew longer, a new set of two collection bags was devised, 24 for liquid wastes and the other for solid wastes. Once used, the bags are sealed and stored for the flight back to the earth, 25 they are disposed of.