Which element does not show catenation property?

Find $A$,$B$ and $C$ in the following reactions:
$NH_3 + A + CO_2 \rightarrow (NH_4)_2CO_3$
$(NH_4)_2CO_3 + H_2O + B \rightarrow 2NH_4HCO_3$
$NH_4HCO_3 + NaCl \rightarrow NH_4Cl + C$

What is the formula of the product formed when $F^{-}$ reacts with enamel on teeth?

In the following reaction,assign the state for the underlined atom in the product of complete hydrolysis at $R.T.$:
$\underline{S}OF_4 + H_2O \longrightarrow H_2SO_4 + HF$

The $[SiO_4]^{4-}$ unit has a tetrahedral structure. Which silicate is formed by sharing three oxygen atoms per $[SiO_4]^{4-}$ unit?

Mention the uses of pure forms of germanium and silicon.