$P_{4}O_{10}$ reacts with water to produce

$NH_3 + O_2 \xrightarrow[\Delta]{Pt} A + H_2O$ ;
$A + O_2 \to B$ ;
$B + O_2 + H_2O \to C$
$A, B$ and $C$ are:

Which halide of nitrogen is least basic?

Under which conditions can active nitrogen be obtained from $N_2$ gas using an electric spark?

When $NH_3$ reacts with $Cl_2$,which of the following statements are correct?
$(a)$ $NCl_3$ is formed when $Cl_2$ is in excess.
$(b)$ $N_2$ is formed when $Cl_2$ is in excess.
$(c)$ $N_2$ is formed when $NH_3$ is in excess.
$(d)$ $NCl_3$ is formed when $NH_3$ is in excess.

How many allotropic forms of black phosphorus exist?