What is the author’s main idea in the passage?

Câu hỏi: Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 39 to 43
There have been a lot of scientific studies into what makes a friendship. It seems today that ideas of what friendship is are also changing. A study in 1993 at the University of Oxford showed that people could only maintain 150 relationships. However, with the explosion of social media since then, many people now have over 300 people who they think of as friends. Some people think that these friends are not real friends, but others believe that social media has helped us to expand and keep our friendships because we have more time and opportunities.
Another change from the past is that people don't stay in the place where they were born. They go to different cities and countries for education and jobs. People are in general more geographically mobile nowadays. This means that we have more chances to meet and make friends with people from different cultures and different backgrounds. People still often make friends at college who remain friends for life, but making new friends at work is more difficult. People now have less security at work and this also means that they find it harder to build new relationships.
As people get older, they sometimes also have less free time. They become busier with their jobs and families and have less time to spend with friends. Friendships can be very
different when we are at different ages. Young children often choose their friends because of convenience, for example, they go to the same school, they live near each other or their parents are friends. As we get older, friendships are more connected with having similar interests and opinions.
One thing is definitely true though, it doesn't matter how old we are or how many friends we have on social media, friendship is good for our health. People who spend time
with friends have fewer mental health problems and are generally happier and in a better physical state of health than people who spend their free time alone.
What is the author’s main idea in the passage?
The word or phrase which best fits the gap (39) is...
A. The older they get, the happier they become
B. The ways to make friends at college and at work
C. Social media and friendship are good for our health
D. Social media and the meaning of friendship