The main buffer system of the human blood is

Question

$\begin{array}{1 1} a.H_2CO_3 - HCO_3 \\ b. H_2CO_3 - CO_3^{2-} \\ c. CH_3COOH - CH_3COO- \\ d. NH_2CONH_2 - NH_2CONH^+ \end{array} $