An organic compound contains C, H, N, S and Cl. For the detection of chlorine, the sodium extract of the compound is first heated with a few drops of concentrated HNO3 and then AgNO3 is added to get a white ppt. of AgCl. The digestion with HNO3 before the addition of AgNO3 is
A.to prevent the formation of NO2
B.to create a common ion effect
C.to convert CN- and S2- to volatile HCN and H2S, or