2 ml each concentrated $\text{HC}l,{\text{HNO}}_3$ and a mixture of concentrated HCl and concentrated ${\text{HNO}}_3$  in the ratio of 3 : 1 were taken in test tubes labelled as A, B and C. A small piece of metal was put in each test tube. In test tube A and B metal not dissolved but the metal got dissolved in test tube C respectively. The metal could be