4.设 sim N(0,1), ϕ(x)为X的分布函数,则对任一实数a,有 (-a)= ()-|||-A. dfrac (1)(2)-Phi (a) B. dfrac (1)(2)+Phi (a) C. https:/img.cdnjtzy.com/zyb_d7b9bdc48af6087bef17bf5f1ad67a13.jpg-(1)(a) D. https:/img.cdnjtzy.com/zyb_d7b9bdc48af6087bef17bf5f1ad67a13.jpg+(1)(a)