$K_a$ values for acids $H_2SO_3$, $HNO_2$, $CH_3COOH$, and $HCN$ are respectively $1.3 \times 10^{-2}$, $4 \times 10^{-4}$, $1.8 \times 10^{-5}$, and $4 \times 10^{-10}$. Which of the above acids produces the strongest conjugate base in aqueous solution?