`Fe^(3+)` can be used for coagulation of `As_2S_3` sol.
(R) `Fe^(3+)` reacts with `As_2 s_3` to give `Fe_2S_3`.
A. IF both (A) and (r) are correct and (r) is the correct explanation for (a).
B. If both (a) and (r) are correct but (r) is not the correct explanation for (a).
C. IF (a) is correct but (r) is incorrect.
D. If (a) is incorrect but (r) is correct.
