Black men earn an average of 21% less than white men. A labor economist who wants to investigate whether the differences in earnings indicate labor-market discrimination against black men should consider whether black and white men have similar

(i) years of work experience.

(ii) years of education.

(iii) occupations.

(iv) quality of education.

a. (i) and (ii) only

b. (i), (ii), (iii), and (iv)

c. (iii) and (iv) only

d. (ii) only