题目

选词填空with while who allow through developInternet is educational . Many people think children should    to surf the Internet as they wish . In this way ,they can look    different subjects on different kinds of websites according to their own interest . But in my opinion ,children should always be watched   surfing the internet .Many websites have information and pictures that children should not see .There are many things that children don't understand .All of these may have some bad influence on children's     Also , it is da如图，四棱柱ABCD—A1B1C1D1的底面边长和侧棱长都等于2，平面A1ACC1⊥平面ABCD，∠ABC＝∠A1AC＝60°，点O为底面对角线AC与BD的交点.（Ⅰ）证明：A1O⊥平面ABCD；（Ⅱ）求二面角D—A1A—C的平面角的正切值.