The correct order of reducing abilities of hydrides of group $15$ elements is:

White phosphorus reacts with thionyl chloride to form $PCl_3$ along with $A$ and $B$. $A$ and $B$ respectively are

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$ : $PH_3$ has lower boiling point than $NH_3$.
Reason $(R)$ : In liquid state $NH_3$ molecules are associated through van der Waals forces,but $PH_3$ molecules are associated through hydrogen bonding.
In the light of the above statements,choose the most appropriate answer from the options given below:

What are the acid anhydrides of $H_3PO_4$ and $HPO_3$ respectively?

Which of the following dissolves in hot conc. $NaOH$ solution?

$A$ gas at low temperature does not react with most compounds. It is almost inert and is used to create an inert atmosphere in bulbs. The combustion of this gas is exceptionally an endothermic reaction. Based on the given information,we can conclude that the gas is