The number$(s)$ of $-OH$ group$(s)$ present in $H_3PO_3$ and $H_3PO_4$ is/are

Which of the following oxides of nitrogen is the anhydride of nitrous acid?

$HCl$ reacts with $NH_3$ to produce white fumes of which compound?

Nitrogen forms $N_2$,but phosphorus prefers to exist as $P_4$ rather than $P_2$. Which of the following is the reason for this?

Orthophosphorus acid on disproportionation gives $PH_3$ and another oxoacid of phosphorus '$X$'. The basicity of $X$ is

Phosphorus forms a number of oxoacids. Out of these oxoacids,phosphinic acid $(H_3PO_2)$ has strong reducing properties. Write its structure and also write a reaction showing its reducing behavior.