单项选择题

Questions 4 to 7 are based on the following conversation. At the end of the conversation, you will be given 20 seconds to answer the questions.
Now listen to the conversation.

According to Kellerman, why are there so many unhappy adults in the world

A.Because they are not brighter than others.
B.Because they don’t have good education.
C.Because they are not doing the things they are best at.
D.Because they are not duly encouraged in life.