Colemanite is

The correct orders among the following are
$A$. Atomic radius $: B < Al < Ga < In < Tl$
$B$. Electronegativity $: Al < Ga < In < Tl < B$
$C$. Density $: Tl < In < Ga < Al < B$
$D$. $1^{st}$ Ionisation Energy $: In < Al < Ga < Tl < B$
Choose the correct answer from the options given below $:$

The hardest substance amongst the following is

Explain the following:
$(1)$ $BF_3$ does not hydrolyze.
$(2)$ Why does the element silicon not form a graphite-like structure,whereas carbon does?

Which one of the following molecular hydrides acts as a Lewis acid?

$AlCl_3$ fumes in moist air because.......