Suppose Y is the annual income, X is the number of years of education, and D is a dummy variable equaling 1 when a person is male. In a linear regression of Y on X and D, we estimate a coefficient of 3 for X and -4 for D. Then:
a. The effect of being female is to lower income by 4.
b. The effect of being male is to raise income by 4, holding education constant.
c. The effect of being female is to raise income by 4, holding education constant.
d. The effect of being female is to raise income by 4.