The stable bivalency of $Pb$ and trivalency of $Bi$ is

Nitrogen is liberated by the thermal decomposition of only:

When $NH_3$ is passed over heated $CuO$,what is it oxidized to?

$N_2$ forms $NCl_3$,whereas $P$ can form both $PCl_3$ and $PCl_5$. Why?

Which of the following oxyacids has an $X-O-X$ linkage?

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.