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

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, dihydrogen phosphate?

A) $K _ { a _ { 1 } }$
B) $\frac { 1 } { K _ { \mathrm { a } _ { 1 } } }$
C) $\frac { K _ { w } } { K _ { \mathrm { a } _ { 1 } } }$
D) $K _ { W } \times K _ { a _ { 1 } }$
E) $\frac { K _ { a _ { 1 } } } { K _ { W } }$