Which of the phosphorus oxoacids can create a silver mirror from $AgNO_3$ solution?

The total number of paramagnetic gaseous products formed in all the following reactions $[A + B + C ]$:
$(A)\ NH_4 NO_3 \xrightarrow{\Delta} \text{Products}$
$(B)\ 3 NaNO_2 + 3 H_2 SO_4 \longrightarrow \text{Products}$
$(C)\ Pb(NO_3)_2 \xrightarrow{673 \ K} \text{Products}$

Ammonia gas can be collected by the displacement of

Which of the following acids forms two series of salts?

The metal which displaces hydrogen from a boiling caustic soda solution is

What is the number of hydroxyl groups in pyrophosphoric acid?