以下条件中能成为平面α∥平面β的充分条件的是（　　）

A．存在一直线l，l∥α，l∥β

B．存在一平面γ，γ∥α，γ∥β

C．存在一直线l，l⊥α，l∥β

D．存在一平面γ，γ⊥α，γ⊥β

Mr. Smith would have come to see you if it had been possible, but he __________ so busy.

A．was

B．had been

C．has been

D．were