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。
答對率僅 12%，是全站最難的一題。
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）。很少人學過它也可以當從屬連接詞，意思是「一旦…就」，後面接完整子句。看到句子需要連接詞時，反而完全聯想不到這個字——這正是本題答對率只有 12% 的原因。
once 當連接詞是「一旦…就…」，重點在「條件成立」——那個門檻一跨過，後面的結果就跟著來：Once you understand the rule, the rest is easy.（一旦懂了規則，其餘就簡單了）。
兩者常被當成同義詞，語意重心其實不同：as soon as 強調的是「時間上緊接著」，前一件事一發生，後一件立刻發生（As soon as we hear any news, we'll call you. 一有消息我們馬上通知你）；once 強調的是「條件成立」，那件事只要成立，結果就會發生。最能看出差別的是這一句：My boss is a nice man once you get to know him.（一旦你了解他，我老闆人很好）——這裡換成 as soon as 就不通，因為「了解一個人」不是瞬間發生的動作，它是一個條件、一道門檻。本句 only climbing once women reach 50 就是「一直要到女性年滿五十，快樂程度才開始回升」。
答案其實寫在下一句：Men, on the other hand, have the lowest point in their 40s, going up again when they reach 50.（男性則是四十幾歲最低落，到五十歲才又回升）——作者描述男性時用 when they reach 50，描述女性時用 once women reach 50，同一個意思換了一個連接詞。前後對照，答案立刻現形。
unless 是「除非」，會變成「除非女性年滿五十，否則才回升」，語意打結；before 是「在…之前」，時間順序剛好顛倒——低潮是持續「到」五十歲，不是五十歲「之前」才結束；since 當連接詞是「自從」或「因為」，兩義放進來都不通。
once + S + V（一旦…就，重條件）、as soon as（一…就，重時間）、the moment (that)（一…就）、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.