Which of the following statements is correct regarding the diffusion capacity of gases?

• A
  The diffusion capacity of $CO_2$ is much higher than that of $O_2$.
• B
  The diffusion capacity of $O_2$ is much higher than that of $CO_2$.
• C
  The diffusion capacity of $O_2$ and $CO_2$ is equal.
• D
  None of the above.