JAMB Chemistry 2010 Question 31

To a solution of an unknown compound, a little dilute tetraoxosulphate (VI) acid was added with some freshly prepared iron (II) tetraoxosulphate (VI) solution. The brown ring observed after the addition of a stream of concentrated tetraoxosulphate (VI) acid confirmed the presence of

1. A.CO₃²⁻
2. B.Cl⁻
3. C.SO₄²⁻
4. D.NO₃⁻