The least stable hydride is:

Identify the correct statements from the following.
$(i)$ Quartz is a piezoelectric material.
$(ii)$ All group $14$ tetrachlorides except $CCl_4$ are easily hydrolyzed in water.
$(iii)$ The $C-C$ bond distance within the layer of graphite is $154 \ pm$.
$(iv)$ $SiO_2$ is soluble in aqueous $HCl$ solution.

The stability of dihalides of $Si, Ge, Sn$ and $Pb$ increases in the order:

$SiCl_4$ on hydrolysis forms '$X$' and $HCl$. Compound '$X$' loses water at $1000^{\circ} C$ and gives '$Y$'. Compounds '$X$' and '$Y$' respectively are:

Identify $X, Y$ and $Z$ in the following reaction: $2 CH_3Cl + X \xrightarrow[570 \ K]{Y} Z$

Carborundum is