When You Have an Omitted Variable Problem, the Assumption That $E \left( u _ { i } \mid X _ { i } \right) = 0$

When you have an omitted variable problem, the assumption that $E \left( u _ { i } \mid X _ { i } \right) = 0$ is violated. This implies that

A) the sum of the residuals is no longer zero.
B) there is another estimator called weighted least squares, which is BLUE.
C) the sum of the residuals times any of the explanatory variables is no longer zero.
D) the OLS estimator is no longer consistent.