What are the number of $P - O - P$ bridges in phosphorus pentoxide $(P_4O_{10})$ and phosphorus trioxide $(P_4O_6)$ respectively?

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

What is the chemical formula of sodium pyrophosphate?

Which of the following represents the correct order of acidic strength?

Which of the following tendencies remains unchanged on going down in the nitrogen family (Group-$15$)?

Which of the following is not correct?