The low point seems to last longer for women—throughout their 30s and 40s, only climbing 15 women reach 50.
答案：(A) once
once 在此是從屬連接詞「一旦…就」，引導時間副詞子句：女性的低潮要一直持續到五十歲才開始回升；unless（除非）語意相反，before（在…之前）時間順序顛倒，since（自從、因為）也不合。
once 在你腦中大概只有兩個身分：副詞「曾經」（I once lived in Japan.）與「一次」（once a week），兩個都是國中學過的。它還有第三個身分——從屬連接詞「一旦…就」，後面接完整子句——課本著墨少，所以句子需要連接詞時不容易想到它。
先看空格後面：women reach 50 是主詞加動詞的完整子句，需要有東西把它接上主要子句，所以這一格要填連接詞。這一步就把 once 的「曾經」與「一次」排除了——那兩個身分是副詞，帶不了子句。確定要連接詞之後再看語意方向：主要子句說女性的低潮橫跨三十幾與四十幾歲，only climbing 表示要到某個時間點才開始回升，所以連接詞要表達「這個條件成立，結果才發生」。once 當連接詞正是這個意思：Once you understand the rule, the rest is easy.（一旦懂了規則，其餘就簡單了。）驗算在下一句。作者接著寫男性：Men, on the other hand, have the lowest point in their 40s, going up again when they reach 50.——同一個「到五十歲才回升」的意思，寫男性時用 when，寫女性時用 once。前後對照，方向就確定了。順帶把 once 與 as soon as 分清楚，兩者常被當成同義詞。as soon as 重的是時間緊接：As soon as we hear any news, we'll call you.（一有消息就通知你。）once 重的是條件成立：My boss is a nice man once you get to know him.（一旦你了解他，我老闆人很好。）後面這句換成 as soon as 就不通，因為「了解一個人」不是瞬間發生的事，是一道門檻。
三個都是連接詞，句法上都塞得進去，要靠語意刷掉。unless 是「除非」，會讀成「除非女性年滿五十，否則才回升」，條件與結果打結。before 是「在…之前」，時間順序顛倒——低潮是持續「到」五十歲，不是五十歲「之前」就結束。since 當連接詞有「自從」與「因為」兩義：「自從女性年滿五十」要配完成式，本句沒有；「因為女性年滿五十」把時間點說成原因，與 only climbing 的語氣對不上。
once + S + V（一旦…就，重條件）、as soon as（一…就，重時間）、the moment (that)（一…就）、by the time …（到…的時候）、once in a while（偶爾）、at once（立刻）。
Recent studies show that levels of happiness for most people change throughout their lives. In a British study between 1991 and 2003, people were asked how satisfied they are 11 their lives. The resulting statistics graph shows a smile-shaped curve.
Most of the people 12 happy and become progressively less happy as they grow older. For many of them, the most miserable period in their life is their 40s. 13, their levels of happiness climb.
Furthermore, it seems that men are slightly happier on average than women in their teens, but women bounce back and overtake men 14 in life. The low point seems to last longer for women—throughout their 30s and 40s, only climbing 15 women reach 50. Men, on the other hand, have the lowest point in their 40s, going up again when they reach 50.