Which of the Following Best Explains Why the Most Rapid

Which of the following best explains why the most rapid increase in a person's wages usually occurs at younger ages,giving a concave shape to the age/earnings profile?

A) On-the-job training is free.
B) On-the-job training usually declines with age.
C) People who have more human capital are more likely to acquire more on-the-job training.
D) People with children cut back their hours of work.