题目

将正方形ABCD沿对角线BD折成一个120°的二面角,点C到达点C1,这时异面直线AD与BC1所成角的余弦值是(    )A.              B.                C.                 D. 答案：解析:由题意易知∠ABC1是AD与BC1所成的角,解△ABC1,得余弦为.选D.答案:DThe book, which took him most of his life to write, is said ________ ever since. A．to have published B．to be published C．to have been published D．having been published