Among the following, which is most likely an instance of discrimination? 

a. A male orthopedic surgeon earns more than a female pediatrician. 

b. The university president's male secretary earns more than the economics department's female secretary.

c. A white worker earns more than a black worker performing the same job at the same level.

d. Women earn less than men, on average.

e. All of the above represent discrimination.