A student was given a solid containing a mixture of nitrate salts. The sample completely dissolved in water, and upon addition of dilute HCl , no precipitate formed. The pH was lowered to about 1 and H2S was bubbled through the solution. No precipitate formed. The pH was adjusted to 8 and H2S was again bubbled in. This time, a precipitate formed. Which compounds might have been present in the unknown? Cu(NO3)2 Mn(NO3)2 KNO3 Zn(NO3)2 Bi(NO3)3 Ca(NO3)2 Cd(NO3)2 Cr(NO3)3