α、β、γ为两两不重合的平面，l、m、n为两两不重合的直线，给出下列四个命题

①若α⊥γ，β⊥γ，则α∥β

②若m∥β，n∥β，则α∥β

③l⊂α，α∥β，则l∥β

④若α∩β=γ，β∩γ=m，γ∩α=n，l∥m，则m∥n

其中正确命题的个数为 ______．

--- How long have you worked here?

--- ________.

A．Five years ago

B．For five years ago

C．Since five years

D．Since five years ago