A.Many students can’t get updated online informtion. B.Many students do not make the most use of printed books. C.Many students can’t judge the quality of online information. D.Many students do not trust the informtion found online.
【单选题】给出如下三个命题: ①若“p且q”为假命题,则 p 、 q 均为假命题; ②命题“若 x ≥2且y≥3,则 x +y≥5”的否命题为“若 x <2且y<3,则 x +y<5”; 学 ③四个实数a、b、c、d依次成等比数列的必要而不充分条件是ad=bc; ④在△ 中,“ ”是“ ”的充分不必要条件. 其中不正确的命题的个数是