The $C-C$ bond length is maximum in?

Which one of the following is correct about the stability of the given ions?

$Si_2O_7^{6-}$ anion is obtained when

$P_2O_5$ is used extensively as a

Match the items in Column $I$ with its main use listed in Column $II$.

Column $I$	Column $II$
$A$. Silica gel	$i$. Transistor
$B$. Silicon	$ii$. Ion-exchanger
$C$. Silicone	$iii$. Drying agent
$D$. Silicate	$iv$. Sealant

Identify the correct statements among the following:
$a$) $C_{60}$ molecule has $12$ five-membered rings and $20$ six-membered rings.
$b$) $H_2CO_3 / HCO_3^-$ buffer system helps to maintain $pH$ of blood between $7.26$ and $7.42$.
$c$) Graphite is used as a dry lubricant in machines running at high temperatures.