Which one of the following cannot form an amphoteric oxide?

Which element forms unstable trihalides?

Three allotropes $(A)$,$(B)$ and $(C)$ of phosphorus in the following change are respectively
$A \xrightarrow[1200 \ atm]{470 \ K} B$
$A \xrightarrow[CO_2 - atm]{570 \ K} C$

Which of the following is the formula for metaphosphoric acid?

On heating,lead$(II)$ nitrate gives a brown gas $(A).$ The gas $(A)$ on cooling changes to a colourless solid/liquid $(B).$ $(B)$ on heating with $NO$ changes to a blue solid $(C).$ The oxidation number of nitrogen in solid $(C)$ is :

$H_3PO_2$ $\xrightarrow{140^{\circ}C}$ $A$ $\xrightarrow{250^{\circ}C}$ $B$ $\xrightarrow{316^{\circ}C}$ $C$
Compound $(C)$ is