A、The economic changes. B、The advanced technology. C、Smaller families. D、High level of self-esteem. A

Teenagers have fewer friends than 20 years ago, research has shown—despite the increasing popularity of social media. A study carried out by experts at the University of Queensland in Australia, found teens felt less lonely than two decades ago—but that they have poorer social networks. (16) A survey of more than 285,000 U. S. high school students carried out between 1991 and 2012 found young people had fewer friends with whom to interact, but less desire for more friends. (17) The Globe and Mail quotes the findings of the research, printed in the Personality and Social Psychology Bulletin, which suggests economic changes may have prompted increased individuality among young people.
    The authors behind the study, wrote, "Greater economic opportunities offer individuals more latitude to manage their own money, decide whom to date and decide whom to marry, reducing the influence of kin and giving people more autonomy, which may increase individualism. Economic changes lead to increased individuality, which could lead to decreasing interest in friends, increasing self-reliance, increasing self-esteem and decreasing loneliness. "
     (18) Lead researcher, Dr. David Clark said further research on cultures outside of the U. S. would help determine if modernization is the root cause of the observed results. He said, "If other cultures show the same pattern of reduced loneliness in the face of poorer social networks, this would support the idea that modernization is responsible. " The findings come after a series of studies, which have shown that the growing use of social networking sites has had a negative effect on the quality of people’s friendships.
16. What finding has been made through the survey of U. S. high school students?
17. What is likely to be the cause of increased individuality among young people?
18. What further study does the lead researcher say is needed for drawing the conclusion?