If you can't think of anything that would have injured your nail, consider the fact that nails grow very slowly, so the injury 18 weeks before the spots ever appeared.
答案：(D) may have occurred
【時態】may have occurred 表示對過去事件的推測，意指「傷害可能早在斑點出現前數週就已發生」。might occur／would occur 為對現在或未來的推測，時態不合過去已發生的情境；will have occurred 為未來完成式，與此處回顧過去傷害發生時間的語境矛盾。
四個選項都是「助動詞加動詞」，你會先去分 may 與 might、will 與 would 的強弱差別，在情態上打轉。真正的分水嶺其實不在那裡，而在有沒有 have 加過去分詞。
先找時間標記。句尾寫著 before the spots ever appeared，appeared 是過去式，表示斑點出現已經是過去的事；而受傷又發生在斑點出現之前，那就是比過去更早。掛鉤是國中的 By the time I got there, he had left.，一樣是把時間再往前推一格。不過這兩件事要分清楚：he had left 是過去完成式，單純陳述「更早發生」這個事實；本題還多了一層「推測」，所以不能只換時態。要用「情態助動詞 ＋ have ＋ 過去分詞」這個組合，讓 may 負責推測、have occurred 負責把時間往前推，答案就是 may have occurred（當時可能已經發生了）。同一句前半還有一個現成的示範：If you can't think of anything that would have injured your nail，作者已經用 would have injured 對過去做過一次推測，空格要的是同一件事。答對率四成五，鑑別度 41，分的是有沒有把句尾那個過去式當成時間軸的錨點。
might occur 的 might 確實也能表推測，但沒有 have 加過去分詞就只能講現在或未來可能發生，本句的受傷早已發生。would occur 是站在過去某個時間點往後看，與「比 appeared 更早」的方向相反。will have occurred 是未來完成式，站在未來某個時間點回頭看，本句的時間軸卻整個在過去，先看時間標記就能一刀刷掉。
may／might have ＋ 過去分詞（對過去的推測：當時可能已經…）、would have ＋ 過去分詞（對過去的推測，或與過去事實相反）、occur（發生，不及物）、think of（想到）、consider the fact that …（想想…這個事實）、grow slowly（生長緩慢）
Many people at some point in life have white spots on their fingernails. One of the most common causes for these little white spots is a condition called leukonychia. Although the name sounds pretty serious, the condition typically 16 . And while many people think the white spots are caused by a calcium or zinc deficiency, that's generally not the case.
In reality, these spots most often develop 17 mild to moderate trauma to your nail. If you can't think of anything that would have injured your nail, consider the fact that nails grow very slowly, so the injury 18 weeks before the spots ever appeared. The spots could also be a sign of a mild infection or allergy, or a side effect of certain medications.
19 the source of the injury, these spots typically do not require any treatment and should go away as your nail grows out. And they should not return unless you suffer another injury to a nail. However, this generally 20 when only a single or a few nails are affected. If all of your nails are showing white spots, the leukonychia could be related to another more serious condition such as anemia, cardiac disease, diabetes, or kidney disease.