The correct order of acidic character of the following is:

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

In the following reaction,assign the correct category for the underlined atom based on the product of complete hydrolysis at $R.T.$:
$\underline{Si}F_4 + 2H_2O \longrightarrow H_4SiO_4 + 4HF$

On heating copper nitrate strongly,$.......$ is finally obtained.

The correct statements with respect to halides of group $14$ elements are:
$I$. Both $CCl_4$ and $SiCl_4$ undergo hydrolysis
$II$. $GeX_4$ is more stable than $GeX_2$
$III$. $PbX_4$ is less stable than $PbX_2$
$IV$. Stability of dihalides decreases down the group.

$A$ certain element forms a solid oxide which when dissolved in water forms an acidic solution. The element is: