题目

如图,正方体ABCD—A1B1C1D1的棱长为1,点M是对角线A1B上的动点,则AM+MD1的最小值为A.2                 B.              C.2+              D. 答案：D The water of the lake is so ________ that it looks like glass． [　　] A． calm B． rapid C． silent D． quiet