For each of the following situations in which similar workers are paid different wages, give the most likely explanation for these wage differences.
a. Test pilots for new jet aircraft earn higher wages than airline pilots.
b. College graduates usually have higher earnings in their first year on the job than workers without college degrees have in their first year on the job.
c. Full professors command higher salaries than assistant professors for teaching the same class.
d. Unionized workers are generally better paid than non-unionized workers.