To avoid the precipitation of hydroxides of Zn²⁺,Mn²⁺ and Ni²⁺ along with those of Fe³⁺,Al³⁺ and Cr³⁺ the third group solution should be :
A Concentrated H₂SO₄ is added
B Concentrated HNO₃ is added
C Treated with excess of NH₄OH
D Treated with excess of NH₄Cl