However, this generally 20 when only a single or a few nails are affected.
答案：(C) applies
apply 在此是「適用」：不過這種說法通常只適用於單一或少數指甲受影響的情況；indicates（顯示）、defines（定義）、confirms（證實）語意都不合。
apply 你大概只熟 apply for a job（申請工作），很少看到它單獨用來表示「適用、成立」。四個選項的中文擺在一起都像那麼回事，你會想憑語意挑一個順的，但這一題句法就先淘汰了三個。
先看空格後面有沒有受詞。this generally ___ when only a single or a few nails are affected——空格後面直接接 when 引導的副詞子句，沒有任何名詞當受詞。indicate、define、confirm 三個都是及物動詞，一定要有受詞（indicate a problem、define a term、confirm a booking），少了受詞句子就不完整。apply 則可以不及物，單獨表示「適用、成立」：This rule applies to everyone.（這條規則適用於所有人。）／The same applies here.（這裡也一樣。）這一步就把答案定下來了。再用文意確認方向。本句以 However 起頭，作用是限縮前面那個結論的適用範圍——前面說白點會隨著指甲長出去、不必治療，這一句補上「不過通常只在少數指甲受影響時才成立」，下一句接著講十指全是白點的情況，正好是被排除在外的那一種。驗算法：把 apply 換成 be true 唸一遍——this is generally true when only a few nails are affected，意思不變，代換成立。
三個都是及物動詞，錯在缺受詞，語意也各自不合。indicate 是「顯示、指出」，要說「顯示什麼」，而且顯示的是徵兆與結論的關係，不是適用範圍。define 是「定義」，後面接被定義的字詞，本句沒有東西要下定義。confirm 是「證實」，後面接被證實的說法或安排；而且 However 這個轉折要的是限縮，不是證實，方向也相反。
apply to（適用於）、The same applies.（同理適用）、apply for a job（求職）、apply pressure（施加壓力）、indicate（顯示）、confirm（證實）、application（申請；應用）。
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.