4. An economist uses data to estimate the linear model with intercept
$y_t = \alpha + \beta_1 x_{1t} + \varepsilon_t,$
but the true population model is as below:
$y_t = \alpha + \beta_1 x_{1t} + \beta_2 x_{2t} + u_t.$
Under which condition satisfied, the OLS estimator $\hat{\beta}_1$ is still consistent?      (4)      
(A). \( E(x_{2t}u_t) = 0 \)
(B). \( \alpha = 0 \)
(C). \( E(x_{2t}\varepsilon_t) = 0 \)
(D). \( E(x_{2t}|x_{1t}) = 0 \)
(E). \( E(x_{1t}u_t) = 0 \)