题目

如图，正方体ABCD－A1B1C1D1中，E、F分别为AB、CC1的中点，则异面直线A1C与EF所成角的余弦值是                                                                                                                                                                      (    ) (A)                    (B)                     (C)                          (D种子在浸水和不浸水的情况下，进行呼吸的过程中都产生 [　　] A．酒精B．磷酸C．丙酮酸D．乳酸