By the time he was 23, he had already come up with the idea of what 18 the modern computer—the Turing machine.
答案：(A) would become
【未來式的過去】would become 表示「未來將成為」，站在過去時間點描述後來成真的未來事件（此構想後來發展成現代電腦）。should become（應該成為）、could have become（本可能成為，暗示未實現）、had become（過去完成式，時序矛盾）均不合文意。
空格前一個子句剛用過 had already come up with（過去完成式），對稱的慣性會讓你想再選一次同樣的形態，had become 於是讀起來特別順；句首的 By the time he was 23 又是過去完成式的典型訊號，更加深了這個念頭。答對率只有五成一。
四個選項是同一個動詞 become 配上四種不同的助動詞，但它們問的不是同一件事：would 問的是時間方向，should 問的是語氣，could have 問的是與事實相符與否，had 問的是先後順序。一格要同時判四樣，所以動手之前先把句子的結構定下來，再一項一項刷。先看 what 在子句裡站哪個位置：the idea of what ______ the modern computer，what 是複合關係代名詞（等於 the thing that），它自己就是這個子句的主詞，所以空格填的必定是這個子句的動詞。接著把時間軸排出來：圖靈 23 歲那年（過去）想出了一個構想，那個構想是後來才發展成現代電腦的。站在過去往後看，用的是過去的未來，形態就是 would 加原形。國中的掛鉤是間接引述 He said he would come.（他說他會來）：說的動作在過去，來的動作在那之後。填 would become，讀成「後來會成為現代電腦的那個東西」，時間軸才對得上。這一題的鑑別度只有 23，連高分群考生也在這裡失手，原因就在前一個子句的 had already come up with：對過去完成式越敏感的人，越容易把它當成整句的基準形態照抄一次。但那個過去完成式說的是「想出構想」比 23 歲更早，方向往前；空格這一格要往後看，方向相反，不能照抄。
had become 方向剛好相反，它表示「在 23 歲之前就已經是現代電腦」，但現代電腦是後來的事，時間顛倒了。should become 的 should 帶著建議或義務的口吻，這一句只是在敘述後來發生的事，沒有人在建議什麼。could have become 是「本來有可能成為，但實際上沒有」，與事實不符——同一段後面還寫著 Today, Turing machines are still used in theoretical computation.
would 加原形（站在過去看未來，後來會……）、come up with（想出、提出）、by the time 加子句（到……的時候）、what 等於 the thing that（複合關係代名詞）、could have 加過去分詞（本來有可能……但沒有）、be used in（用於……）
Alan Turing was one of the leading scientific geniuses of the 20th century. Many scholars consider him the father of modern computer science. He was also the man who cracked the 16 uncrackable Enigma code used by Nazi Germany. His code-breaking turned the tide of World War II and helped save two million lives. Nevertheless, 17 people have even heard his name. Turing displayed signs of high intelligence in math and science at a young age. By the time he was 23, he had already come up with the idea of what 18 the modern computer—the Turing machine. Today, Turing machines are still used in theoretical computation. He also proposed the now famous Turing test, used to determine whether a computer exhibits intelligent behavior equivalent to that of a human.
The postwar era, however, was a disaster for Turing. He was gay, which was then a crime in Britain. 19 being hailed as one of the crucial figures in defeating the Nazis, Turing was convicted of "gross indecency." This 20 drove him to commit suicide in 1954, at the age of 41.
Nearly 60 years after his death, Queen Elizabeth II granted Turing a formal pardon for his conviction, upon an online petition signed by prominent scientists and technology leaders around the world.