Quant Doubts

Hi guys. The folowing doubts are from Schweser Book No 1 in Quant section: 1. Page 184: The expected value of the error term, conditional on the independent variable is zero. Why should it be conditional on independent variable? We did not have a similar condition for linear regression (Page 148) 2. Page 202: DW test statistic is approximately equal to 2 if the error terms are homoskedastic and not serially correlated. Why can’t they be heteroskedastic? I don’t think that will create any difference 3. Page 255: Q18: In the solution they have found DOF as 100 Shouldn’t it be: 99? As there are 102 months -> 101 observations --> (101-2) DOF Thanks!