Which of the following hydride is covalent and polymeric :-

$\mathrm{X}\underset{{\mathrm{N}}_2}{\overset{∆}{\to }}\mathrm{Y}\stackrel{{\mathrm{H}}_2\mathrm{O}}{⟶}\mathrm{Z}$ (a colourless gas) $\stackrel{{\mathrm{CuSO}}_4}{\to }\mathrm{T}$ (blue colour) :

Then, substances$\mathrm{Y}$ and $\mathrm{T}$ are -

If $\mathrm{A}$ is the metalic salt, then the white $\mathrm{ppt}.$ of $\mathrm{D}$ must be of