题目

三棱锥A—BCD的棱长全相等, E是AD中点, 则直线CE与直线BD所成角的 余弦值为（    ）        A．        B.         C．        D． 答案：D_______ by thick bamboo and high forest, the ancient city hadn’t been discovered until the 1990s.A．SurroundingB．SurroundC．SurroundedD．Having surrounded