题目

While e-book sales this year have declined, it is still important for us to remember that the figures are still higher than ______ five years ago. A . one B . that C . those D . they 答案：C已知正四棱柱ABCD-A1B1C1D1中，AA1=2AB，E为AA1中点，则异面直线BE与CD1所成角的余弦值为（    ）