Chọn đáp án đúng và giải thích

Scientists study (18) ____________. Today, our ideas of friendship are changing. In the past, people could only keep 150 friendships. Now, with social media, some have over 300 "friends”. Some say these aren't real friends, but others believe social media helps us (19) ____________.

            People move more today for jobs and education. (20) ____________. College friends can last a lifetime, but making work friends can be harder because jobs are less secure.

            (21) ____________, we often have less free time due to work and family. Friendships change with age too. Young children play with friends who live nearby or go to the same school. Adults tend to choose friends with similar interests.

            (22) ____________, friendship is healthy. People with strong friendships are happier and healthier than those who spend free time alone.
Question 20.   

A. It allows us to meet people from different cultures and backgrounds.

            B. Allowing us to meet people from different cultures and backgrounds.

            C. Which allows us to meet people from different cultures and backgrounds.

            D. This allows us to meet people from different cultures and backgrounds.