Three separate samples of a solution of a single salt gave these results. One formed a white ppt. with excess ammonia solution, one formed a white ppt. with dil. NaCl solution and one formed a black ppt. with $H_2S$. The salt could be:

Question

(a) $AgNO_3$
(b) $Pb(NO_3)$
(c) $Hg(NO_3)_2$
(d) $MnSO_4$