设 $F_{1}(x)$ 与 $F_{2}(x)$ 为两个分布函数, 其相应的概率密度 $f_{1}(x)$ 与 $f_{2}(x)$ 是连续函数, 则必为概率密度的是 ( ) (A) $f_{1}(x)f_{2}(x)$ . (B) $2f_{2}(x)F_{1}(x)$ $(\mathrm{C})f_{1}(x)F_{2}(x)$ $(\mathrm{D})f_{1}(x)F_{2}(x) + f_{2}(x)F_{1}(x).$