Which of the following is not a pseudohalide?

Explain the following:
$(a)$ $CO_2$ is a gas whereas $SiO_2$ is a solid.
$(b)$ Silicon forms $SiF_6^{2-}$ ion whereas corresponding fluoro compound of carbon is not known.

With respect to graphite and diamond,which of the statement$(s)$ given below is (are) correct?
$(A)$ Graphite is harder than diamond.
$(B)$ Graphite has higher electrical conductivity than diamond.
$(C)$ Graphite has higher thermal conductivity than diamond.
$(D)$ Graphite has higher $C-C$ bond order than diamond.

$Na_2CO_3$ reacts with $SO_2$ in an alkaline medium to form:

What is the product formed in the reaction $SnO_2 + HCl$ (excess)?

In the carbon family,elements other than carbon do not form $p\pi-p\pi$ bonds because their atomic radii are very ........