A:160°
B:150°
C:140°
D:120°
茂名市人口大约有680万人,用科学记数法可表示为( )
January is the ____(one) month of a year.
①若α∥β,α∥γ,则β∥γ ②若α⊥β,m∥α,则m⊥β
③若m∥n,n⊂α,则m∥α ④若m⊥α,m∥β,则α⊥β
其中正确命题的序号是( )