In a regression equation predicting income (measured in $1000s) with education (measured in years), the intercept (a) equals 1.5 and the regression coefficient (b) equals 4. Which of the below is the correct interpretation of the intercept (a)?

a. at zero years of education, income equals $1500
b. when income equals zero, education equals 1.5

c. for every additional year of education, income increases $4000

d. for every four years of education, income increases $1000