4.已知 NH3 的 Kb= 1.6 ×10−3，CH3NH2 的 Kb= 3.9 ×10−4，C2H5NH2 的 Kb= 4.7×10−4，請問三者的鹼性強弱順序，下列何者正確？
(A) NH3＞CH3NH2＞C2H5NH2
(B)C2H5NH2＞CH3NH2＞NH3 
(C) CH3NH2＞NH3＞C2H5NH2
(D) NH3＞C2H5NH2＞CH3NH2