在直三棱柱A1B1C1 ABC中，∠BCA＝90°，点D1，F1分别是A1B1，A1C1的中点

在直三棱柱A₁B₁C₁ ABC中，∠BCA＝90°，点D₁，F₁分别是A₁B₁，A₁C₁的中点，BC＝CA＝CC₁，则BD₁与AF₁所成角的余弦值是(　　)

A. B.

C. D.

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在直三棱柱A1B1C1 ­ ABC中，∠BCA＝90°，点D1，F1分别是A1B1，A1C1的中点

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在直三棱柱A1B1C1 ABC中，∠BCA＝90°，点D1，F1分别是A1B1，A1C1的中点