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。
答對率 21%。
alone 你記得的是「獨自」（She lives alone.），那個 alone 修飾動詞。本題的 alone 放在名詞 Chance 後面，意思是「僅僅、光靠」＝ only，但位置在名詞之後，這個用法很陌生。另一個陷阱是拼字：alone 與 alike 只差兩個字母，趕時間很容易看錯。
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）。它一定放名詞後面，不能像 only 那樣放前面。
本句用 however 轉折：前一句說偶然的發現促成了盤尼西林的問世，本句說光靠運氣還不夠，下一句 The scientist must have a prepared and open mind 補上另一個必要條件。三句一氣呵成。
alone（獨自／僅僅）、let alone（更不用說）、leave sb alone（別打擾某人）。
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）。
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.