${\mathrm{H}}_3{\mathrm{PO}}_3$ can be represented by structures 1 and 2 shown below. 

These two structures cannot be taken as the canonical forms of the resonance hybrid, because  : 

1. The positions of the atoms have changed.

2. The positions of the atoms are constant.

3. H₃PO₃ does not show resonance.

4. Two hydrogen atoms are missing.

The number of resonating structures exhibited by CO₃^2- ion are :

1. 2

2. 3

3. 4

4. 5