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
此處站在過去（圖靈23歲時）的時間點展望「後來會發展成」現代電腦，須用過去式的未來（would + 原 V）表示「未來將成為」；should become 語氣為義務建議，不符；could have become／had become 皆為完成式，表示「本應成為」或「已經成為」，與此處單純敘述未來發展的語意不符。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
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。
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，「後來會成為現代電腦的那個東西」，時間軸才對得上。
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。它之所以誘人，是因為前一個子句剛用過 had already come up with，形態一致讀起來很順，但那個過去完成式是在說「想出構想」比「23 歲」更早，與空格這一格要往後看的方向不同，不能照抄。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 ＋ 原形（站在過去看未來，後來會……）、come up with（想出、提出）、by the time ＋ 子句（到……的時候）、what 等於 the thing that（複合關係代名詞）、could have ＋ 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.