After the Examination Is Completed,the Patient Asks the Nurse Why

After the examination is completed,the patient asks the nurse why Chadwick's sign occurs during pregnancy.What would the nurse explain as the cause of Chadwick's sign?

A) Enlargement of the uterus
B) Progesterone action on the breasts
C) Increasing activity of the fetus
D) Vascular congestion in the pelvic area