The incentre of the triangle formed by the lines $x\cos \alpha +y\sin \alpha =\pi$ ,$x\cos \beta +y\sin \beta =\pi$, $x\cos \gamma +y\sin \gamma =\pi$ is $(\alpha ,\beta )$ then $\alpha +\beta$ = 

• A

  0

• B

  1

• C

  2

• D

  4