Which one of the following is formed (mainly) when red phosphorus is heated in a sealed tube at $803 \,\,K$?

On heating with concentrated $NaOH$ solution in an inert atmosphere of $CO_{2}$,white phosphorus gives a gas. Which of the following statements is incorrect about the gas?

Two oxides of nitrogen,$NO$ and $NO_2$,react together at $253 \ K$ to form a compound of nitrogen $X$. $X$ reacts with water to yield another compound of nitrogen $Y$. The shape of the anion of $Y$ is:

Which of the following is the most suitable drying agent for ammonia gas?

Which of the following statements is correct regarding $H_3PO_3$ and $H_3PO_4$?

White phosphorus is heated with concentrated $NaOH$ in $CO_2$ atmosphere to form a gas $A$ and compound $B$. When $A$ is bubbled into aqueous $CuSO_4$ solution,copper phosphide and $C$ are formed. $B$ and $C$ are respectively: