A:70
B:67
C:63
D:50
There____ ____interesting in the newspaper.
如图,A1B1C1-ABC是直棱柱,∠BCA=90°,点D1,F1,分别是A1B1,A1C1的中点.若BC=CA=CC1,则BD1与AF1所成角的余弦值为( )