Solid PbCl2 and solid Hg2Cl2 are contained in separate test tubes. From the following list of reagents, which pair of reagents would allow one to distinguish one test tube from the other?
a. Hot water and K2CrO4
b. HCl and HNO3
c. K2CrO4 and HNO3
d. Ag+ and Pb2+