How many $H-$ atoms are directly bonded to the phosphorus atom in hypophosphorous acid?

Phosphorous and phosphoric acids are,respectively, . . . . . . acids.

Of the different allotropes of phosphorus,the one which is most reactive is

Mercury $(II)$ chloride solution on reaction with gaseous ammonia forms:

How can the brown color of $HNO_3$ be removed?

Choose the correct statement$(s)$ among the following.
$A$. $SnCl_2 \cdot 2 H_2O$ is a reducing agent.
$B$. $SnO_2$ reacts with $KOH$ to form $K_2[Sn(OH)_6]$.
$C$. $A$ solution of $PbCl_2$ in $HCl$ contains $Pb^{2+}$ and $Cl^{-}$ ions.
$D$. The reaction of $Pb_3O_4$ with hot dilute nitric acid to give $PbO_2$ is a redox reaction.