The mark of heroes is not necessarily the result of their action, but 24 they are willing to do for others and for their chosen cause. 25 they fail, their determination lives on for others to follow.
答案：(A) what
what 引導名詞子句作主詞補語，「而是他們願意為他人做什麼」；who（指人，這裡指的是「什麼事」不是「誰」）、those（不構成這樣的主詞補語子句）、where（地方）都不合。
what 一看到就想成疑問詞「什麼」，於是這個句子讀起來像在問問題。其實 what 更常見的身分是「本身自帶先行詞的關係代名詞」，等於 the thing which 或 the things which，引導一個名詞子句，整包當名詞用。
用數子句成分的方法，這一招最可靠。空格後面是 they are willing to do for others，主詞 they 有了，動詞 are willing to do 也有了，可是 do 是及物動詞，它的受詞不見了。缺的那個受詞就是 what 本身。還原給自己看：they are willing to do something for others，把 something 抽出來換成 what 提到前面。再看整句的結構：The mark of heroes is not necessarily the result of their action, but ___ they are willing to do...。not A but B 把兩邊並列，A 的位置是 the result of their action（一個名詞片語），B 的位置就必須同樣是名詞性的東西，而且接在 be 動詞 is 後面當主詞補語。what 引導的名詞子句正好符合。同樣靠「數子句成分」定案的還有 97 年第 27 題、98 年第 23 題與 105 年第 21 題，是同一個判準，值得整組一起練。
who 指人，但空格處要的是「他們願意做的事」不是「誰」；而且 who 若當關係代名詞，後面的子句就該缺主詞，這裡主詞 they 明明在。those 是指示代名詞，後面要再接名詞或關係子句，湊不出一個完整的名詞子句。where 指地方，這句裡沒有任何地點的線索。
be willing to V（願意做）、what matters（重要的是）、not A but B（不是 A 而是 B）、not necessarily（未必、不見得）、the mark of sth（……的特徵、標記）、a chosen cause（自己選定的志業）。
There are two kinds of heroes: heroes who shine in the face of great danger, who perform an 21 act in a difficult situation, and heroes who live an ordinary life like us, who do their work 22 by many of us, but who 23 a difference in the lives of others.
Heroes are selfless people who perform extraordinary acts. The mark of heroes is not necessarily the result of their action, but 24 they are willing to do for others and for their chosen cause. 25 they fail, their determination lives on for others to follow. The glory lies not in the achievement but in the sacrifice.