Consider a simple linear regression model, y = β0+ β1x + u, . What does the zero conditional mean assumption imply? a. The estimated average value of β1 is zero. b. The estimated average value of β0 is zero. c. The expected value of the explained variable, y , is zero, regardless of what the value of the explanatory variable, x , is. d. The expected value of the error term, u , is zero, regardless of what the value of the explanatory variable, x, is.