A consumer's spending is widely believed to be a function of their income. To estimate this relationship, a university professor randomly selected 19 of his students and collected information on their spending (Y, in dollars) and income (X, in dollars) patterns in week 6 of the semester. Assuming a linear relationship between Y and X, the professor used the least-squares method and found that the Y intercept = 20.90 and the slope = 0.66. Based on this information, the Y intercept should be interpreted as: a. When a student's weekly spending equals zero, the mean value of his/her weekly income is estimated to be $21.56. b. When a student's weekly spending equals zero, the mean value of his/her weekly income is estimated to be $20.90. c. When a student's weekly income equals zero, the mean value of his/her weekly spending is estimated to be $20.90. d. When a student's weekly income equals zero, the mean value of his/her weekly spending is estimated to be $21.56