已知m，n表示不同直线，α，β，γ表示不同平面．

①若α∩β=m，n⊂α，n⊥m，则α⊥β

②若α⊥β，α∩γ=m，β∩γ=n，则n⊥m

③若α⊥β，α⊥γ=m，β∩γ=m．则m⊥α

④若m⊥α，n⊥β，m⊥n，则α⊥β

以上四个命题中真命题为______．

Remember that there is still one point ______ we must make clear at the conference tomorrow.

A．where

B．why

C．when

D．that