Which from following buffers is used to maintain the pH of human blood naturally?

Calculate the solubility of sparingly soluble salt BA in $\mathrm{mol} \mathrm{dm}^{-3}$ at 300 K if its solubility product is $4.9 \times 10^{-9}$ at same temperature.

Calculate the pH of buffer solution containing 0.04 M NaF and $0.02 \mathrm{~M~HF}\left[\mathrm{pK}_a=3 \cdot 142\right]$.

What is the pOH of millimolar solution of $\mathrm{Ca}(\mathrm{OH})_2$ ?