Phosphoric Acid Is a Triprotic Acid, Ionizing in the Following $\leftrightarrows$

Multiple Choice

Phosphoric acid is a triprotic acid, ionizing in the following sequential steps:
H₃PO₄ + H₂O $\leftrightarrows$ H₂PO₄^- + H₃O⁺ $K _ { a _ { 1 } }$
H₂PO₄^- + H₂O $\leftrightarrows$ HPO₄^2- + H₃O⁺ $K _ { a _ { 2 } }$
HPO₄^2- + H₂O $\leftrightarrows$ PO₄^3- + H₃O⁺ $K _ { a _ { 1 } }$
What is the K_b expression for the base, sodium phosphate?

A) $K _ { a _ { 1 } }$
B) $$\frac { 1 } { K _ { a _ { 3 } } }$$
C) $$\frac { K _ { W } } { K _ { a _ { 3 } } }$$
D) $\bar { K } _ { \mathrm { W } } \times \bar { K } _ { \mathrm { a } _ { 3 } }$
E) $\frac { K _ { W ^ { 3 } } } { K _ { \mathrm { a } _ { 1 } } \times K _ { \mathrm { a } _ { 2 } } \times K _ { \mathrm { a } _ { 3 } } }$