Chance 20 , however, may not be enough to make key discoveries like this.
答案：(A) alone
alone 放在名詞之後表示「僅僅、光靠」：然而光靠機運，未必足以促成這樣的重大發現；alike（相似地）、above（在上方）、ahead（在前方）語意都不合。
alone 你記得的是「獨自」（She lives alone.），那個 alone 修飾動詞、放在句尾。本題的 alone 緊跟在名詞 Chance 後面，意思變成「僅僅、光靠」，等於 only——但位置在名詞之後，這個用法課本著墨少。
先抓骨幹。主詞是 Chance，主要動詞是 may not be enough，however 是插入的轉折語。空格緊貼在主詞後面，前後沒有動詞可修飾，所以它修飾的是 Chance 這個名詞，要填的是一個能後位修飾名詞的字，不是一般的方位副詞。alone 正是這一類：它放在名詞後面表示「唯獨、光是」，而且只能放後面，不能像 only 那樣挪到前面。Money alone can't buy happiness.（光靠錢買不到幸福。）／Time alone will tell.（唯有時間能證明。）驗算法：把 alone 換成 only 並移到句首唸一遍——Only chance may not be enough to make key discoveries like this.（光靠機運未必足以促成這樣的重大發現。）意思不變，代換成立，方向就對了。文意也對得上：本句用 however 轉折，前一句講的是偶然帶來的發現，下一句 The scientist must have a prepared and open mind（科學家必須有備而來、心胸開放）補上另一個必要條件——三句合起來就是「光有運氣不夠，還要有準備」。
三個選項都以 a- 開頭，字形接近，但都不能後位修飾名詞。alike 是「相似地、同樣地」，用來說兩者一樣（The twins dress alike.），本句沒有兩個對象可比。above 與 ahead 是方位副詞，一個表示上方、一個表示前方，都用來講位置或順序，機運沒有上下前後可言。分不出來時回到位置這一關：空格修飾的是名詞，這三個修飾的都是動詞。
X alone（光靠 X）、Time alone will tell.（唯有時間能證明）、let alone（更不用說）、leave sb alone（別打擾某人）、alike（同樣地）、serendipity（機緣巧合的發現）。
Scientific discovery can take various forms. One of them is "serendipity," the luck of finding valuable things unintentionally. Serendipity was at work when Alexander Fleming discovered penicillin, the antibiotic miracle.
Fleming had long been known for having an untidy laboratory. One morning in 1928, this 16 proved very fortunate. It was his first day coming back to work after a long vacation. Before the vacation, he 17 in the sink a number of petri dishes in which he had been growing bacteria. While he was sorting through the long 18 dishes of germs that morning, he observed that some of the dishes were contaminated with a fungus, which had ruined his experiment. He was about to 19 the dishes, but he noticed that in one dish, the bacteria had failed to grow in the area around the fungus. This accidental finding gave rise to subsequent research that led to the discovery of penicillin—a drug that has since saved millions of lives.
Chance 20 , however, may not be enough to make key discoveries like this. The scientist must have a prepared and open mind to detect the importance of the unforeseen incident and to use it constructively.