A salt, when warmed with zinc powder and an NaOH solution, gives a gas that turns a filter paper soaked with an alkaline solution of K2[HgI4] brown. The salt responds to the brown ring test when acetic acid is used in place of sulphuric acid. The anion present in the salt is:
a. NO3−
b. NO2−
c. Br−
d. None of these