Oxidation number of P in pyrophosphoric acid is:

1.  +5
2.  +2
3.  +3
4.  +4

A compound having two direct bonds between P and H atoms is:

1. $H_{3}P{O}_2$

2. $H_{3}P{O}_3$

3. $H_{3}P{O}_4$

4. $H_4{P}_2{O}_7$

Nitrogen form $N_2$, but phosphorous  form $P_2$, it's at a time convert in $P_4$, reason is:

1. Triple bond present between phosphorous atoms. 
2. $\mathrm{p}\mathrm{\pi }-p\mathrm{\pi }$ bonding is weak in phosphorous.
3. $\mathrm{p}\mathrm{\pi }-p\mathrm{\pi }$ bonding is strong in phosphorous.
4. Multiple bonds easily form phosphorous.