直三棱柱ABC-A₁B₁C₁中，若∠BAC=90°，AB=AC=AA₁，则异面直线BA₁与AC₁所成的角等于 [ ]

A．30°

B．45°

C．60°

D．90°

All the athletes competed _____ each other for gold medals in the Olympic Games.

A．against

B．for

C．between

D．over