4.设随机变量X服从正态分布N(0,1),对给定的 alpha (0lt alpha lt 1), 数ua满足 Xgt {u)_(alpha )} =alpha .-|||-若 |X|lt x =alpha , 则x等于 ()-|||-(A) dfrac (a)(2) (B) _(1)-dfrac (alpha )(2) (C) dfrac (1-a)(2) (D) _(1-1)