Before the vacation, he 17 in the sink a number of petri dishes in which he had been growing bacteria.
答案：(B) had piled
【時態】had piled 為過去完成式，表示「休假前」他已將培養皿堆放好，此動作發生在另一過去事件（回來工作）之前，故用過去完成式。would pile（過去習慣性動作，語意不合）、was piling（過去進行式，未強調先後順序）、might have piled（推測語氣，不合肯定敘述）均不合文意。
四個選項都是助動詞加動詞，很容易一個一個去想這個語氣通不通。這一類時態題有更快的路：先找句子裡的時間標記，時間標記決定時態，不必憑語感。
時間標記就寫在句首：Before the vacation（在休假之前）。整段敘事的基準時間是過去——前一句才說 It was his first day coming back to work after a long vacation.（那是他休完長假回來上班的第一天）。「休假之前」比這個過去的基準點更早，所以要用過去完成式，也就是 had piled。國中掛鉤是 By the time I got there, he had left.：兩件事都發生在過去，先發生的那一件往前推一格用過去完成式。同一句後半的 in which he had been growing bacteria 用的也是同一個時間層，兩者互相印證。
might have piled 形態上也帶著 have 與過去分詞，最容易被誤認；但 might have ＋ 過去分詞 是對過去的推測（當時也許堆了），而本文是在敘述一件確實發生過的事，不是在猜測。was piling 是過去進行式，強調當時正在進行的動作，但這裡要的是在那之前就已經完成的事。would pile 是過去的未來或過去的習慣性動作，兩者都對不上這個時間點。鑑別度 40，比同篇其他題略低，原因是這一格的判準很單純：認得先發生的事要往前推一格就選得出來，不太需要回頭讀上下文推理。
過去完成式（表示比過去某個時間點更早發生）、by the time ＋ 過去式子句（等到……的時候）、might have ＋ 過去分詞（當時也許……，對過去的推測）、pile sth in／on（把……堆在……）、a number of（若干、一些）、petri dish（培養皿）
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.