$A$ graph plotted between $pO_{2}$ and the percentage saturation of $Hb$ with $O_{2}$ is known as:

• A
  $O_{2}$ association curve
• B
  $CO_{2}-O_{2}$ dissociation curve
• C
  $O_{2}$ dissociation curve
• D
  $CO_{2}-O_{2}$ association curve