单项选择题

A new economics paper has some old-fashioned advice for people navigating the stresses of life: Find a spouse who is also your best friend. Social scientists have long known that 1 people tend to be happier, but they debate whether that is because marriage causes happiness or simply because happier people are more 2 to get married. The new paper, 3 by the National Bureau of Economic Research, controlled for pre-marriage happiness levels. It 4 that being married makes people happier and more satisfied 5 their lives than those who remain single—particularly during the most stressful periods, like 6 crises.
Even as fewer people are marrying, the disadvantages of remaining single have broad 7 . It"s important 8 marriage is increasingly a force behind inequality. 9 marriages are more common among educated, high-income people, and increasingly out of reach for those who are not. That divide appears to 10 not just people"s income and family stability, but also their happiness and stress levels.
A quarter of today"s young adults will have never married by 2030, which would be the highest 11 in modern history, according to Pew Research Center. 12 both remaining unmarried and divorcing are more common among less-educated, lower-income people. 13 , high-income people still marry at high rates and are less likely to divorce.
Those whose lives are most difficult could 14 most from marriage, according to the economists who wrote the new paper, John Helliwell and Shawn Grover. "Marriage may be most important when there is that stress in life and when things are going 15 ," Mr. Grover said.
16 marital happiness long outlasted the honeymoon period. 17 some social scientists have argued that happiness levels are innate, so people return to their natural level of well-being 18 joyful or upsetting events, the researchers found that the benefits of marriage persist. One 19 for that might be the role of friendship within marriage. Those who 20 their spouse or partner to be their best friend get about twice as much life satisfaction from marriage as others, the study found.

A.Unless
B.Hence
C.Until
D.Though