2 mL each of conc. HCl, `HNO_(3)` and a mixture of conc. HCl and conc. `HNO_(3)` in the ratio of3 : 1 were taken in test tubes labelled as A, B and C. A small piece of metal was put in each test tube. No change occurred in test tubes A and B but the metal got dissolved in test tube C. The metal could be