— The light in his room is still on. Do you know _________?

— To prepare for the coming exam, I think.

A．if he works hard

B．why he stays up so late

C．why is he so busy

D．when he will stop working

翼尖涡引起的在机翼翼展范围内的气流（）运动，翼展之外的气流向（）运动.

A.向上、向下

B.向上、向上

C.向下、向上