Hot concentrated sulphuric acid on reaction with which one of the following elements,produces two gaseous products?

The catenation tendency of $C, Si$ and $Ge$ is in the order $Ge < Si < C$. The bond energies (in $kJ \ mol^{-1}$) of $C-C, Si-Si$ and $Ge-Ge$ bonds, respectively, are:

Concentrated hydrochloric acid,when exposed to air,sometimes produces a white cloud of smoke. What is the explanation for this?

In the following reaction,identify the nature of the product formed by the complete hydrolysis of the underlined atom at $R.T.$:
$\underline{P}_4O_6 + H_2O \longrightarrow H_3PO_3$

$SiCl_4$ on hydrolysis gives:

The plague of tin is the: