黄河是我国的第一大河流。()

A．could be simplified to if  is a square matrix.
B．All above are correct.
C．could be simplified to  if there is no endogeneity problem and  is taken as `instrumental variables' that .
D．is unbiased and consistent if standard assumptions are satisfied in Chapter 7, in particular, the instrumental variables are correlated with the endogenous regressors but uncorrelated with the error term.