In the Ostwald process for the manufacture of $HNO_3$,which of the following is used as a catalyst?

Nitrous oxide $(N_2O)$ is:

$PCl_{5} + P_{4}O_{10} \xrightarrow{\Delta } X$. The product $(X)$ is an oxygen-containing compound. How many moles of an oxyacid are formed on the complete hydrolysis of $1 \text{ mole}$ of $(X)$?

What is produced when $Ag$ is heated with concentrated $HNO_3$?

When $NH_3$ is passed over heated $CuO$,it gets oxidized to form .................. .

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: