${\mathrm{Mg}}_2{\mathrm{C}}_3$ on hydrolysis in the presence of acid gives a major product $\left(\mathrm{X}\right)$. The product is:

Give reasons for the following:

($\mathrm{a}$) ${\mathrm{CCl}}_4$ is immiscible in water, whereas ${\mathrm{SiCl}}_4$ is easily hydrolysed.

Give reasons for the following:

($\mathrm{b}$) Carbon has a strong tendency for catenation compared to silicon.

Consider the following statements:

$\mathrm{I}$: In diamond, each carbon atom is linked tetrahedrally to four other carbon atoms by ${\mathrm{sp}}^3$ bonds.
$\mathrm{II}$: Graphite has planar hexagonal layers of carbon atoms held together by weak Van der Waals forces.
$\mathrm{III}$: Silicon exists only in diamond structure due to its tendency to form ${\mathrm{p}}_{\mathrm{\pi }}-{\mathrm{p}}_{\mathrm{\pi }}$ bonds to itself. 

Which statement (s) is/are correct?