Which of the following determines the fractional concentration of CO after 10 seconds of breath holding (F_ACO_T)?

A) (F_AHe/F_IHe) $\times$ F_ICO
B) Volume inspired divided by F_AHe/F_IHe
C) Estimation of the ratio F_ICO/F_AHe
D) Measurement of CO from an alveolar sample