What is the number of hydroxyl groups in pyrophosphoric acid?

${\mathrm{HNO}}_3+{\mathrm{P}}_4{\mathrm{O}}_{10}\to {\mathrm{HPO}}_3+{\mathrm{N}}_2{\mathrm{O}}_5$

The role of ${\mathrm{P}}_4{\mathrm{O}}_{10}$ in the given reaction is:

Identify incorrect statements:

a. Moist ammonia gas can be dried by using anhydrous ${\mathrm{CaCl}}_2$.

b. ${{\mathrm{NO}}_3}^{-}$ and ${{\mathrm{PO}}_3}^{3-}$ are isostructural.

c. Phosphine is a stronger base than ammonia because phosphorus atom is less electronegative than nitrogen.

d. Sodium hypophosphine is an acidic salt.