Which of the following statement$(s)$ is/are correct?
$(a)$ High concentration of carbonic anhydrase in RBCs.
$(b)$ High concentration of carbonic anhydrase in blood plasma.
$(c)$ Every $100 \ ml$ of deoxygenated blood delivers approximately $4 \ ml$ of $CO_2$ to the alveoli.
$(d)$ $20-25\%$ of $CO_2$ is carried by hemoglobin as oxyhemoglobin.

• A
  $a$ and $c$
• B
  $a$ and $d$
• C
  $a, b, c$ and $d$
• D
  Only $a$