However, their demand is high because many people buy them to tell the world that they have money, 28 as conspicuous consumption in economics.
答案：(B) termed
過去分詞 termed 作補語，被動語意「這（種行為）在經濟學裡被稱為炫耀性消費」，補充說明前面整個現象的名稱；term（原形動詞，不合這個分詞補語結構）、terms（限定動詞，句子已有主要動詞不需要另一個）、was termed（另立子句雖然文法通，但這裡分詞構句更自然精簡）都不如termed。
看到空格前面有逗號、後面有 as，很多人想補一個完整的子句進去，於是挑了 was termed 或 terms。也有人把 term 當成名詞「術語」，就填了原形。
先數主要動詞。逗號前面的 many people buy them to tell the world that they have money 已經是完整的句子，主要動詞是 buy。逗號後面如果再放一個有時態的動詞（terms、was termed），就會變成兩個句子只靠逗號相連而沒有連接詞——英文不允許這樣寫。這一點和 101 年第 17 題（some of which）是同一個原理，中文可以一路逗號到底，英文不行。所以空格要的是非限定的形式。接著決定主動或被動：被稱呼的是前面那件事，動作是被動，所以用過去分詞 termed。還原成完整子句驗算：which is termed conspicuous consumption in economics（這在經濟學上被稱為炫耀性消費），把 which is 一起省略掉，就只剩下 termed。term 在這裡是動詞「把…稱為」，用法是 term A B，被動就是 A is termed B；加上 as 寫成 term A as B 也是實際存在的說法（劍橋詞典兩種都收）。這種「關係代名詞加 be 動詞一起省略」留下過去分詞的形狀，你其實看過最短的版本：a broken window（被打破的窗戶）、the songs played on the radio（在廣播上播放的歌曲）。
term：原形動詞，既不能當主要動詞，也不能就這樣接在逗號後面。terms：有時態的動詞，會造成一個句子出現兩個主要動詞；語意上也變成「錢稱呼…」，講不通。was termed：本身文法沒錯，但前面沒有連接詞，同樣是兩句只靠逗號相連；要用它得寫成 which was termed 或 and it was termed。
term A as B（把 A 稱為 B）、conspicuous consumption（炫耀性消費）、in economics（在經濟學上）、be known as（被稱為…）、be referred to as（被稱為…）、tell the world that（向世人宣告…）。
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.