Calculate the equilibrium constant for the dissociation of

Calculate the equilibrium constant for the dissociation of all protons from H3PO4, given Kb1 = 2.4Ã10-2, Kb2 = 1.6Ã10-7, and Kb3 = 1.4Ã10-12. (Temperature is 298.15 K.) A) 2.4Ã10-2 B) 4.2Ã10-13 C) 1.9Ã10-22 D) 7.2Ã10-3 E) 6.3Ã10-8 
Answer is C but I’m not sure how to approach it