In the United States, the mean annual income for blacks (1) is smaller than for whites (2),

In the United States, the mean annual income for blacks (μ1) is smaller than for whites (μ2), the mean number of years of education is smaller for blacks than for whites, and annual income is positively related to number of years of education. Assuming that there is no interaction, the difference in the mean annual income between whites and blacks, controlling for education, is

(a) Less than μ2 − μ1.

(b) Greater than μ2 − μ1.

(c) Possibly equal to μ2 − μ1.