But pineapples can also be a fashionable fruit: Pineapple leaves can 21 a substitute for leather.
答案：(D) serve as
【句型】serve as 字詞搭配「作為/充當」；bring along（帶著）、turn out（結果是）、account for（解釋/占比）都不合。
serve 你最熟的是「供應餐點、服務」，看到 serve a substitute 會覺得怪怪的。這裡考的是 serve 另一個常考的意思：「充當、作為」，後面固定接 as。
空格後面是 a substitute for leather（皮革的替代品），一個名詞片語，所以空格要放的是能接名詞、而且語意是「當作」的動詞片語。serve as 正好符合：主詞 Pineapple leaves（鳳梨葉）和後面的 a substitute（替代品）是同一件東西，中間是「充當」的關係。換成你更熟的說法驗算一次：Pineapple leaves can serve as a substitute for leather. 等於 Pineapple leaves can be used as a substitute for leather.（鳳梨葉可以被當成皮革的替代品使用），意思一樣，可見 serve as 就是 be used as 的同義說法。
bring along（隨身帶著）的受詞是可以帶著走的東西，「鳳梨葉帶著一個替代品」語意不通。turn out（結果是）後面要接 to be 或 that 子句，例如 It turned out to be a mistake.，不能直接接名詞。account for（解釋原因；占某個比例）和「當作替代品」無關。
serve as（充當、作為）、act as（擔任、充當）、function as（發揮……的功能）、be used as（被當成……使用）、a substitute for（……的替代品）、substitute A for B（用 A 代替 B）、bring along（隨身帶著）、turn out to be（結果是）、account for（解釋；占……比例）。
The pineapple, a delicious tropical fruit, has been valued for centuries not only for its distinct and unique taste, but also for its miraculous health and medical benefits. But pineapples can also be a fashionable fruit: Pineapple leaves can 21 a substitute for leather.
The idea was developed when a Spanish designer travelling to the Philippines observed a traditional Filipino shirt 22 together with the fibers of pineapple leaves. After five years of research, she created from pineapple leaves a material which, like real leather, can be used for making bags, shoes, and 23 textile products. It is an eco-friendly and biodegradable fiber.
This eco-friendly leather has clear 24 for the environment, compared to real leather and synthetic leathers. It requires fewer chemicals, making it safer for the workers in factories. Its manufacture also leaves a smaller carbon footprint. In addition, the leftover material following the removal of fibers can even be used as a natural fertilizer back in the pineapple fields. 25 its low cost, this innovative material is already being used by many leading fashion companies to make their products.