$({\mathrm{CH}}_3{)}_3\mathrm{N}$ is pyramidal and acts as a Lewis base wbereas $({\mathrm{SiH}}_3{)}_3\mathrm{N}$ is planar and has a very little basic character. Comment on the statement.

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?