A multiple linear regression model is to be constructed to determine if there is a relationship between a dependent variable (y) and two independent variables (x1 and x2). A random sample of size n has been collected and the values of x1i, x2i and yi for i = 1, 2, ..., n have been recorded. The residuals (ei) in this analysis are defined as the difference between the observed values of y and the values of y predicted by the regression equation. Select the condition that is one of the assumptions of a valid multiple linear regression model:
a) the relationship between the dependent and independent variables is linear
b) the residuals are constant
c) the independent variables are independent of the dependent variable
d) the relationship between the dependent and independent variables is quadratic