11.设随机变量X的密度函数为f(x ),则 ((X)^2)= (-|||-)。-|||-A (int )_(-infty )^-infty xf(x)dx-|||-B (int )_(-infty )^+infty (x)^2f(x)dx-|||-C (int )_(-infty )^+infty x(f)^2(x)dx-|||-D (int )_(-infty )^+infty ((x-E{(x))^2f(x)dx