The key to a healthy lifestyle is to keep our stress level 15 .
答案：(A) balanced
keep + O + OC 結構中，受詞補語需用形容詞化的過去分詞 balanced，因為「壓力程度（stress level）」與「平衡（balance）」之間是被動關係——壓力程度是「被」保持在平衡狀態，而非主動去平衡什麼，故用過去分詞 balanced 作受詞補語；balancing（現在分詞，主動）語意不合；balances 為動詞變化形式，文法上不能直接作受詞補語；to balance（不定詞）不用於此句型。
keep 這個字，你的印象大概停在 keep ＋ V-ing（keep talking 一直講、keep running 一直跑），所以看到 balancing 就覺得眼熟，很容易直接選它。但那個 keep 後面是直接接動名詞、中間沒有受詞；本句的 keep 後面卡著一個受詞 our stress level，走的是完全不同的句型。答對率四成四，多半就折在這裡。
本句是 keep ＋ 受詞 ＋ 受詞補語，受詞是 our stress level，空格是補語，用來說明這個受詞被保持在什麼狀態。國中學過的 The news made me happy.（那個消息使我開心）就是同一個結構：made 是動詞、me 是受詞、happy 是受詞補語。接下來只要判斷主動還是被動：壓力程度不會自己去平衡別的東西，而是被保持在平衡的狀態，所以用過去分詞 balanced。還原成完整的說法驗算一次：keep our stress level balanced ＝ keep our stress level (which is) balanced。
balancing 是現在分詞，方向是主動的，代進去等於說壓力程度自己在平衡什麼東西，語意不通。balances 是限定動詞，這個句子裡已經有 is 當主要動詞，不可能再放一個有時態的動詞進來。to balance 是不定詞，keep 不走 keep ＋ 受詞 ＋ to V 這條路。鑑別度 55，分開的是有沒有停下來判斷 stress level 與 balance 之間的主動被動關係。
keep ＋ 受詞 ＋ 受詞補語（使……保持在某種狀態）、keep sth balanced（讓……保持平衡）、the key to ＋ 名詞（……的關鍵）、a healthy lifestyle（健康的生活型態）、stress level（壓力程度）、low motivation（動力低落）
We all know that too much stress is not good for our health, but too little is not ideal, either. While 11 stress can be dangerous to the body, short-term stress is actually healthy.
Short-term stress triggers the production of protective chemicals in our body and strengthens the body's defenses. 12 our body is in a vulnerable situation, a burst of stress will quickly mobilize the body's own repair system to defend the damaged areas. This 13 us from physical discomfort and sickness.
Small amounts of stress hormones may even sharpen our memory. A recent study found that when rats were forced to swim—an activity that places them under stress for a short while—they remembered their way through mazes far 14 than rats that were in a relaxed state.
The key to a healthy lifestyle is to keep our stress level 15 . Too much stress will make us cranky and sick. Too little stress, on the other hand, will lead to boredom and low motivation.