The number of $P-OH$ bonds present in pyrophosphoric acid and hypophosphoric acid is respectively

The total number of $\sigma$ and $\pi$-bonds in pyrophosphoric acid are respectively

Which of the following statements about phosphine $(PH_3)$ is/are correct?
$(a)$ It has a rotten egg-like smell.
$(b)$ In aqueous solution and in the presence of light,it yields red phosphorus and hydrogen.
$(c)$ It explodes with $Br_2$.
$(d)$ In its pure form,it is non-inflammable.

When orthophosphoric acid is heated to $600\,^oC$,it forms .................. .

$NO_2$ is a mixed oxide. This is proved by the fact that it reacts with $H_2O$ and then with $NaOH$ to form:

$X$ reacts with dilute nitric acid to form 'laughing gas'. What is $X$?