On average, 50-year old workers are paid several times more than workers in their teens and twenties. Which of the following options is the most likely explanation for that huge difference

in average earnings? 

a. Older workers have more human capital and higher MRPs.

b. Employers engage in widespread discrimination against younger workers.

c. Young people lack information about the existence of the high-paying jobs occupied by older workers.

d. Older workers receive compensating differences because they do jobs that are more risky than the jobs done by younger workers.