Metaphosphoric acid has the formula

Match the following List-$I$ (Acid) with List-$II$ (Reagents used for preparation):
| List-$I$ (Acid) | List-$II$ (Reagents used for preparation) |
| :--- | :--- |
| $A. H_3PO_2$ | $I. \text{Red } P_4 \text{alkali}$ |
| $B. H_4P_2O_5$ | $II. \text{White } P_4 \text{alkali}$ |
| $C. H_3PO_4$ | $III. PCl_3; H_3PO_3$ |
| $D. H_4P_2O_7$ | $IV. P_2O_5; H_2O$ |
| | $V. H_3PO_4; \Delta$ |

Nitrogen exists as $N_2$,whereas phosphorus exists as $P_4$. This is because ..........

Which of the following nitrogen trihalides is the most basic?

$H_3PO_4$ $\xrightarrow{250\ ^oC}$ $(X)$ $\xrightarrow{600\ ^oC}$ $(Y)$
$H_3PO_4$ $\xrightarrow{600\ ^oC}$ $(Y)$
$(X)$ and $(Y)$ are:

Which of the following dissolves in hot conc. $NaOH$ solution?