Which of the following oxides of nitrogen is isoelectronic with $CO_2$?

The number of $P-H$,$P-O-P$,$P-O-H$ and $P=O$ bonds in sodium dihydrogen pyrophosphate $(Na_2H_2P_2O_7)$ are respectively:

$PCl_5$ exists but $NCl_5$ does not because:

$Fe$ is made passive by

On heating lead $(II)$ nitrate gives a brown gas $'A'$. The gas $'A'$ on cooling changes to a colourless solid $'B'$. Solid $'B'$ on heating with $NO$ changes to a blue solid $'C'$. Identify $'A'$,$'B'$ and $'C'$,write the reactions involved,and draw the structures of $'B'$ and $'C'$.

In which of the following options do all species contain $X-O-X$ bonds in their structure ($X =$ central atom)?