Values of dissociation constant, $K_a$ are given as follows:

| Acid | $K_a$ |
|------|------|
| HCN | $6.2 \times 10^{-10}$ |
| HF | $7.2 \times 10^{-4}$ |
| $HNO_2$ | $4.0 \times 10^{-4}$ |

The correct order of increasing base strength of the bases $CN^-$, $F^-$ and $NO_2^-$ will be: