Which of the following is a tetrabasic acid?

Which of the following oxides of nitrogen is a blue solid?

Bones glow in the dark because

Assertion $:$ Reducing nature of pnicogen hydrides increases on moving down the group.
Reason $:$ Thermal stability of pnicogen hydrides decreases on moving down the group.

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.

The number of hydroxyl group in pyrophosphoric acid is