Less than 3 percent of the world's water is fresh, and most of 17 is trapped in polar ice or buried underground in springs too deep to reach.
答案：(B) it
空格代替前面提到的不可數名詞 water（單數），most of it 表示「其中大部分」。which 需要關係子句結構，what 不構成完整名詞子句，them 是複數代名詞跟 water 不一致。
看到 most of 後面空一格、再接一個動詞，很容易覺得這裡要放關係詞，於是在 which 和 what 之間猶豫。答對率只有四成二，多數人卡在這一步。
關係詞題一律用代入法：把先行詞代回去，看它在子句裡變成什麼形式。空格指的是前半句的 the world's water。代進去讀成 most of water is trapped in polar ice，這已經是一個完整的句子，不缺任何成分——空格要的只是一個代替 water 的代名詞，不是關係詞。water 是不可數名詞，所以用單數的 it。這是指標勾環：代名詞回指前面出現過的東西，填之前先問「它代替的是哪一個字」。鑑別度只有 26，難題卻分不出高下，原因就在這一步是反射動作——看到 most of 加空格就認定是關係子句題，文法練得越熟的人越容易一頭栽進 which 與 what 的比較，忘了先確認這一句到底缺不缺成分。
which：關係代名詞要接子句，寫法應該是 ..., most of which is trapped ...，前面要有逗號讓它接上先行詞；本句 and 後面是一個獨立的子句，放 which 反而多出一個沒有主要動詞的結構。what：它沒有先行詞可以代，本身要展開成 the thing(s) that，代進去變成 most of the thing that is trapped，與前半句接不起來。them：複數，water 不可數，數不合。
most of it（其中大部分）、less than（少於）、be trapped in（受困於……）、too ... to ...（太……以致於無法……）、fresh water（淡水）、polar ice（極地冰層）。
More than 2.7 billion people will face severe shortages of fresh water by 2025. There is, 16, only a limited amount of water on the planet. Less than 3 percent of the world's water is fresh, and most of 17 is trapped in polar ice or buried underground in springs too deep to reach. Freshwater lakes, rivers and reservoirs may seem 18 but provide just a drop in the bucket.
Even 19 supplies are sufficient or plentiful, they are increasingly at risk due to various forms of pollution.
Fierce national competition 20 water resources has prompted fears that water issues contain the seeds of violent conflict. International cooperation is therefore by all means urgent.