Many mammals fluff up their fur when 19 , to look bigger and thus more dangerous.
答案：(C) threatened
過去分詞 threatened，省略主詞的分詞構句（when [they are] threatened），被動語意「當牠們受到威脅時」；threaten（原形動詞，不合這個省略結構）、threatening（現在分詞，主動語意方向不合，應該是動物「被威脅」）、to threaten（不定詞）都不合。
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四個選項是 threaten 的四種形態，你會想找主詞、判主動被動；但空格前面只有一個 when，看起來什麼都沒有，一時不知從何下手。那個「什麼都沒有」正是線索——它省略了主詞與 be 動詞。
先看結構：Many mammals fluff up their fur when ___ , to look bigger and thus more dangerous.（許多哺乳類在……時會把毛蓬起來，讓自己看起來更大、更危險。）when 後面照理要接一個完整的子句（主詞加動詞），但這裡直接接了一個字——這是省略：當副詞子句的主詞與主要子句相同時，可以把主詞與 be 動詞一起省略。還原之後是 when they are threatened（當牠們受到威脅時），省略 they are 就剩下 threatened。再分主動被動：哺乳類是被威脅的一方，所以用過去分詞 threatened。這一招在題庫裡反覆出現：90 年第 24 題的 until (it is) turned off（直到它被關掉）、還有 While (he was) walking home 這種寫法，機制完全相同。看到 when、while、until、if 後面直接接分詞或形容詞，就先把主詞與 be 動詞補回去。
threatening：現在分詞表主動，變成「當牠們威脅別人時」，方向反了——牠們是因為受到威脅才蓬毛自保。threaten：原形動詞。when 後面要接的是子句，原形動詞既沒有主詞也沒有時態，站不住。to threaten：不定詞表目的，但這裡是時間副詞子句，不是在講目的。
when (they are) threatened（當牠們受威脅時）、fluff up one’s fur（把毛蓬起來）、feel threatened（感到受威脅）、a means of（……的手段）、encounter a predator（遇上掠食者）、thus（因此）。
Goose bumps are one of those fight-or-flight reactions of the sympathetic nervous system. When you experience cold or fear, a nerve reaction is sent to the muscles, which then contract involuntarily. This muscle contraction causes your body hair to stand erect. The skin with standing body hair 16 a goose's skin after its feathers have been plucked—hence the name goose bumps. But 17 is this helpful to us as a response to fear or cold?
Back in the days when our ancestors may have had a lot more body hair, this might have helped to keep them warm or scare an oncoming predator. These days, however, goose bumps are 18 to human beings.
Goose bumps, or the reflex that causes them, also occur in many other mammals. Many mammals fluff up their fur when 19 , to look bigger and thus more dangerous. For example, when a porcupine encounters a predator, its long sharp pointed hairs will stand up as a means of 20 . In animals with a thick hair coat, the rising of hair traps more air between the hairs, providing another layer of insulation to keep them warm.