在长方体ABCD-A₁B₁C₁D₁中，AA₁=AB=2，AD=1，点E、F、G分别是DD₁、AB、CC₁的中点，则异面直线A₁E与GF所成的角是______．

I was drawing a horse _____ the teacher came in.   [ ]

A. while                    

B. as  

C. when                    

D. the moment