C2H5OH+PCl3⟶C2H5Cl+H3PO3
H3PO3+PCl3⟶H4P2O5+HCl
A is H3PO3(Phosphorous acid)
The basicity of A is two
B is H4P2O5(Pyrophosphorous acid)
Structure of H4P2O5 is as given below

Hydrogen which are directly attached to the phosphorous in the above structure are non-ionisable.
The Number of non-ionisable hydrogen atoms are two in the above structure.