7.设F1(x)和F 2(x)都是随机变量的分布函数,f1(x x),f2(x)是相应的概率密度,则下列-|||-必为概率密度的是 ()-|||-A. _(1)(x)(f)_(2)(x) B.2f2(x)F1(x) C.f1(x)F2(x) D. _(1)(x)(F)_(2)(x)+(f)_(2)(x)(F)_(1)(x)