Being a rare natural resource, diamonds are 27 in supply.
答案：(C) limited
語意「鑽石供應『有限』」，limited（有限的）符合稀缺資源的語境；traded（被交易）、weakened（被削弱）、noticed（被注意到）都不合。
四個選項都是過去分詞，代進去形狀一模一樣，只能靠語意判斷。traded 特別誘人，因為鑽石確實是被交易的商品，be traded 文法上也完全通。
決定性的線索就在同一句的開頭：Being a rare natural resource（因為是一種稀有的天然資源）。這是分詞構句表原因，還原成完整子句是 Since diamonds are a rare natural resource。既然稀有，供給當然是「有限的」，所以是 limited。下一句還有第二道保險：However, their demand is high（然而它們的需求很高）。However 要求前後相反——需求高，前面就必須是供給少。這一組「供給少、需求高」正是整篇要解釋的供需道理，也是後面 the scarcity of goods（商品的稀有性）的伏筆。順帶把 in supply 這個說法記起來：be limited in supply（供給有限）、be in short supply（供不應求），supply 在這裡是不可數名詞。
traded：被交易，文法通，但語意既接不上前半的「稀有」，也接不上後半 However 的轉折。weakened：被削弱，英文不用「弱」來形容供給，而且削弱講的是一個變化的過程，這裡說的是長期的狀態。noticed：被注意到，與供需完全無關。
be limited in supply（供給有限）、be in short supply（供不應求）、supply and demand（供給與需求）、a natural resource（天然資源）、demand is high（需求量大）、Being a rare resource（身為一種稀有資源，分詞構句表原因）。
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.