Acidified $K_2Cr_2O_7$ solution turns green when $Na_2SO_3$ is added to it. This is due to the formation of :

Question

( A ) $Cr_2(SO_3)_3$
( B ) $Cr_2(SO_4)_3$
( C ) $CrO_4^{2-}$
( D ) $CrSO_4$