题目

In France it is the__________ to shake hands with people in the office every morning.        A. habit                 B. behavior                  C. custom                   D. act 答案：C如图，正方体的棱长为1，线段上有两个动点E，F，且，则下列结论中错误的是  （   ） A． B． C．三棱锥的体积为定值 D．异面直线所成的角为定值