A Woman in Labour Is Breathing into a Mouthpiece Just

A woman in labour is breathing into a mouthpiece just before the start of her regular contractions.As she inhales,a valve opens,and gas is released.She continues to inhale the gas slowly and deeply until the contraction starts to subside.When the inhalation stops,the valve closes.Which statement is accurate in relation to this scenario?

A) This method is not used much anymore.
B) This method is likely to be used only in the second stage of labour.
C) This describes an application of nitrous oxide.
D) This describes a preparation for Caesarean birth.