The pH of an aqueous solution of sodium hydrogencarbonate is approximately $\frac{1}{2}(\mathrm{p}{K}_{a1}+\mathrm{p}{K}_{a2})$. For $\mathrm{H}{}_2\mathrm{CO}{}_3$, $\mathrm{p}{K}_{a1}=6.35$ and $\mathrm{p}{K}_{a2}=10.33$. The pH is about

1. A$6.35$
2. B$10.33$
3. C$8.34$
4. D$7.00$