Which of the following acids has the weakest conjugate base?
a) NH₃, Ka = 1 x 10⁻³⁶
b) H₂O, Ka = 1 x 10⁻¹⁴
c) H₂S, Ka = 1 x 10⁻⁷
d) H₂Se, Ka = 1 x 10⁻⁴