Elements $X$ and $Y$ belong to Group $15$. The difference between the electronegativity values of $X$ and phosphorus is higher than that of the difference between phosphorus and $Y$. $X$ and $Y$ are respectively:

Which of the following ions does not exist in an aqueous solution?

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

The number of hydroxyl groups in pyrophosphoric acid is ......... .

The correct statement$(s)$ related to oxoacids of phosphorus is(are):
$(A)$ Upon heating,$H_3PO_3$ undergoes disproportionation reaction to produce $H_3PO_4$ and $PH_3$.
$(B)$ While $H_3PO_3$ can act as a reducing agent,$H_3PO_4$ cannot.
$(C)$ $H_3PO_3$ is a monobasic acid.
$(D)$ The $H$ atom of the $P-H$ bond in $H_3PO_3$ is not ionizable in water.

Which of the following hydrides is the least stable?