This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
During pregnancy there is an increased respiratory sensitivity to Carbon dioxide due to higher circulating levels of : 
Select and explain. 
A. Progesterone, B. Estrogen, C. Estriol, D. Prolactin
A: Progesterone
Answer: A: Progesterone 
Explanation: Progesterone