The basic character of hydrides of the $V$-group elements decreases in the order:

$A$ obtained by Ostwald's method involving air oxidation of $NH_3$,upon further air oxidation produces $B$. $B$ on hydration forms an oxoacid of Nitrogen along with evolution of $A$. The oxoacid also produces $A$ and gives positive brown ring test.

$PbO_2$ is obtained from

Which of the following statements is correct for $N_2O_4$?

$A$ group $15$ element forms $d \pi - d \pi$ bond with transition metals. It also forms a hydride,which is the strongest base among the hydrides of other group members that form $d \pi - d \pi$ bonds. The atomic number of the element is $.......$

The molecular formula of an acid of phosphorus is $H_3PO_2$. What are its name and basicity,respectively?