For 0.2 M nitrous acid, HNO₂, in aqueous solution at equilibrium, which is the correct expression for Ka if x represents the concentration of H₃O?
a) Ka = [HNO₂] / [H₃O]
b) Ka = [H₃O] / [HNO₂]
c) Ka = [H₃O]² / [HNO₂]
d) Ka = [HNO₂]² / [H₃O]