For many of them, the most miserable period in their life is their 40s. 13 , their levels of happiness climb.
答案：(A) After that
After that（在那之後）承接前句提到的「40多歲是最痛苦的時期」，表示過了這段時期後快樂程度回升。By that time（到那時為止）、Not for long（不會太久）、Before now（在此之前）都不符合這種「之後」的時間順接語意。
四個選項都在講時間，中文都能說成「那之後」「那時候」，你會憑語感挑一個。
這是邏輯勾環，而且線索一定要跨句才看得到。答對率四成四。前一句是 For many of them, the most miserable period in their life is their 40s（對許多人來說，人生最低潮是四十幾歲）；空格這一句是 their levels of happiness climb（快樂程度開始回升）。回升發生在低潮之後，所以要的是「在那之後」——After that，其中的 that 指的正是前一句那個四十幾歲的低潮期。填進去讀成 After that, their levels of happiness climb。
By that time（到那時為止）把時間停在低潮那一刻，但回升是在低潮之後才發生；Not for long（不會持續太久）本身不是時間的順接，後面該接一個會結束的狀態；Before now（在此之前）指的是到現在為止的過去，與這一段在描述一生的曲線對不上。
after that（在那之後）、by that time（到那時為止）、from then on（從那時起）、not for long（不會持續太久）、the most miserable period（最低潮的時期）、climb（攀升、上升）。
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.