You are answering a USMLE-style medical question. Question: A 22-year-old woman from a rural area who recently discovered she was pregnant is referred for a cardiology consultation due to cyanosis, dyspnea, and a cardiac murmur revealed at the initial prenatal visit. She is gravida 1, para 0 with an estimated gestational age of 19 weeks. She says that the murmur was found in her childhood, and the doctor at that time placed her under observation only. However, she has been lost to follow-up and has not had proper follow up in years. Currently, she complains of dizziness and occasional dyspnea on exertion which has gradually increased during her pregnancy. Prior to her pregnancy, she did not have any symptoms. The vital signs are as follows: blood pressure 125/60 mm Hg, heart rate 81/min, respiratory rate 13/min, and temperature 36.7°C (98.0°F). Her examination is significant for acrocyanosis and a fixed splitting of S2 and grade 3/6 midsystolic murmur best heard over the left upper sternal border. Which of the following physiological pregnancy changes is causing the change in this patient’s condition? Options: A) Increase in heart rate B) Decrease in systemic vascular resistance C) Increase in cardiac output D) Increase in blood volume Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.