In a combination of $NO_3^-, Br^-$ and $I^-$ present in a mixture, $Br^-$ and $I^-$ interfere in the ring test for $NO_3^-$. These are removed by adding a solution of

Question

(a) $AgNO_3$
(b) $Ag_2SO_4$
(c) $Ag_2CO_3$
(d) None of these