7．当锐角时，的值是

C．                                                       D．

A．                                                       B．

6．已知是锐角，且，那么等于

C．                                                    D．

A．                                                             B．

5．下图是一个棱长为的正方体盒子，一只蚂蚁在的中点M处，它到的中点N的最短路线是

C．                                           D．

A．                                              B．