What is the number of hydroxyl groups in pyrophosphoric acid?

The reaction of zinc with dilute and concentrated nitric acid,respectively,produces:

The reaction of $HNO_3$ with $P_4O_{10}$ produces ....

The correct statement with respect to dinitrogen $(N_2)$ is:

In the following reaction,identify the nature of the products formed by the complete hydrolysis of $\underline{P}_4O_8$ at $R.T$:
$\underline{P}_4O_8 + H_2O \longrightarrow H_3PO_3 + H_3PO_4$

The element that can form multiple bonds with itself from the following is