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 was bubbled through the solution. No precipitate formed. The pH was adjusted to 8 and was again bubbled in. This time, a precipitate formed. Which of the following compounds listed below might have been present in the unknown?
