设 $\left(X_1, Y_1\right),\left(X_2, Y_2\right), \cdots,\left(X_n, Y_n\right)$ 为来自总体 $N\left(\mu_1, \mu_2\right.$;
$\left.\sigma_1^2, \sigma_2^2 ; \rho\right)$ 简单随机样本，令 $\theta=\mu_1-\mu_2, \bar{X}=\frac{1}{n} \sum_{i=1}^n X_i$,$\bar{Y}=\frac{1}{n} \sum_{i=1}^n Y_i, \hat{\boldsymbol{\theta}}=\bar{X}-\bar{Y} ，$ 则 $(\quad)$
$\text{A.}$ $E(\hat{\theta})=\theta, D(\hat{\theta})=\frac{\sigma_1^2+\sigma_2^2}{n}$
$\text{B.}$ $\boldsymbol{E}(\hat{\boldsymbol{\theta}})=\theta ， D(\hat{\theta})=\frac{\sigma_1^2+\sigma_2^2-2 \rho \sigma_1 \sigma_2}{n}$
$\text{C.}$ $E(\hat{\theta}) \neq \theta, \quad D(\hat{\theta})=\frac{\sigma_1^2+\sigma_2^2}{n}$
$\text{D.}$ $E(\hat{\theta}) \neq \theta, D(\hat{\theta})=\frac{\sigma_1^2+\sigma_2^2-2 \rho \sigma_1 \sigma_2}{n}$