The oxygen dissociation curve (as percentage saturation versus $p O_2$ of blood) of haemoglobin for a healthy adult individual is shown in the graph.
$\mathbf{P}, \mathbf{Q}$ and $\mathbf{R}$ in the graph, respectively, represent $\mathrm{pO}_2$ of blood in:

• Correct answer
  A

  veins (post-exercise); veins (at rest); arteries
• B

  arteries; veins (at rest); veins (post-exercise)
• C

  arteries; veins (post-exercise); veins (at rest)
• D

  veins (at rest); veins (post-exercise); arteries

Hence, the correct answer is option: 'A'.