7.设 sim N(mu ,(sigma )^2) ,ϕ(x)为标准正态分布函数,则 u-3sigma lt Xlt mu +3sigma = C _.-|||-(A) ϕ(3); (B) https:/img.zuoyebang.cc/zyb_896d45d200d9c002cc7527b00da730ce.jpg-(1)(3) ; (C) (1)(3)-1 ; (D) https:/img.zuoyebang.cc/zyb_896d45d200d9c002cc7527b00da730ce.jpg-2(1)(3)