设连续型随机变量 $X_{1}$ 与 $X_{2}$ 相互独立且方差均存在, $X_{1}$ 与 $X_{2}$ 概率密度分别为 $f_{1}(x)$ 与 $f_{2}(x)$ , 随机变量 $Y_{1}$ 的概率密度为 $f_{Y_{1}}(y) = \frac{1}{2} [f_{1}(y) + f_{2}(y)]$ , 随机变量 $Y_{2} = \frac{1}{2} (X_{1} + X_{2})$ , 则( ) (A) $E(Y_{1}) > E(Y_{2}), D(Y_{1}) > D(Y_{2})$ (B) $E(Y_{1}) = E(Y_{2}), D(Y_{1}) = D(Y_{2})$ (C) $E(Y_{1}) = E(Y_{2}), D(Y_{1}) < D(Y_{2})$ $\left(\mathrm{D}\right) E\left(Y_{1}\right)=E\left(Y_{2}\right), D\left(Y_{1}\right)>D\left(Y_{2}\right)$ . # 二、填空题(本题共6小题，每小题4分，共24分，把答案填在题中横线上.)