Which of the following elements forms a molecule consisting of eight of its own atoms?

Amphibole silicate structure has '$x$' number of corners shared per tetrahedron. The value of '$x$' is

Identify the cyclic silicate ion given in the figure below:

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

In which allotrope of carbon does each carbon atom form four bonds with other carbon atoms?

$A$ black sulphide precipitate when treated with ozone becomes white. The white compound is: