Mercury is the only metal which is liquid at $0^{\circ }\mathrm{C}$. This is due to

1. $\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\mathrm{ZnCrO}.{\mathrm{H}}_2\mathrm{O}$ has yellow colour.

2. ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7+{\mathrm{H}}_2{\mathrm{O}}_2$ gives ${\mathrm{K}}_2{\mathrm{CrO}}_4$.

3. ${\mathrm{Mn}}_3{\mathrm{O}}_4, {\mathrm{Mn}}_2{\mathrm{O}}_3$ and ${\mathrm{MnO}}_2$ are amphoteric in nature.

4. Nichrome contains $25\%$ $\mathrm{Ni}$.

In the reaction, $2{\mathrm{CuCl}}_2+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{SO}}_2\to \mathrm{A}+{\mathrm{H}}_2{\mathrm{SO}}_4+2\mathrm{HCl}$ 

$\mathrm{A}$ is