In other words, the scarcity of goods is 29 causes humans to attribute value.
答案：(A) what
what 引導名詞子句作 is 的補語，「物品的稀缺性是『使人類賦予價值的原因』」；which（需要明確先行詞，這裡沒有）、why（不合這個主詞補語子句結構）、how 都不合。
空格前面是 is、後面是動詞 causes，很多人想填一個關係代名詞去修飾前面的 goods 或 scarcity，就挑了 which。答對率只有 43%，是這一年最低的一題。難點在於這裡的 what 不是疑問詞「什麼」，而是一個自己帶著先行詞的關係代名詞，等於 the thing that。
先看句子缺什麼。the scarcity of goods is ______ causes humans to attribute value. 主詞是 the scarcity of goods，動詞是 is，所以 be 動詞後面缺的是主詞補語，而補語的位置要放名詞或名詞子句。再看空格後面：causes 是動詞，前面沒有主詞。所以空格必須同時做兩件事——對外當這個名詞子句的頭，對內當 causes 的主詞。能一個字兼兩個身分的就是 what。把它拆開唸一次就懂了：what 等於 the thing that，整句是 the scarcity of goods is the thing that causes humans to attribute value（商品的稀有性就是那個讓人類賦予價值的東西）。這也正是 which 不行的原因：which 需要先行詞擺在它前面，而這裡 be 動詞後面是空的，沒有東西給它修飾。你國中學過的短句版本：This is what I want.（這就是我要的東西）等於 This is the thing that I want.，同一個結構，只是那裡的 what 當受詞、這裡當主詞。
which：關係代名詞，必須有先行詞；放在 be 動詞後面沒有東西可修飾，整句會缺補語。why：只能接在 the reason 後面（the reason why），而且 why 之後要接完整的句子，這裡的 causes 缺主詞。how：同樣是副詞性的，既不能當 causes 的主詞，也不能單獨當 be 動詞的補語。
what（…的東西，等於 the thing that）、in other words（換句話說）、the scarcity of（…的稀有性）、cause somebody to do something（使某人做某事）、attribute value to something（賦予某物價值）、This is what I want.（這就是我要的）。
Which is more valuable? Water or diamonds? Water is more useful to mankind than diamonds, and yet 26 are costlier. Why? Called the diamond-water paradox, this is a classic problem posed to students of economics.
The answer has to do with supply and demand. Being a rare natural resource, diamonds are 27 in supply. However, their demand is high because many people buy them to tell the world that they have money, 28 as conspicuous consumption in economics. In other words, the scarcity of goods is 29 causes humans to attribute value. If we 30 surrounded by an unending abundance of diamonds, we probably wouldn't value them very much.
Hence, diamonds carry a higher monetary value than water, even though we find more use for water.