You are answering a USMLE-style medical question. Question: A 20-year-old healthy man is running on a treadmill for the last 30 minutes. He is in good shape and exercises 3 times per week. He takes no medications and denies smoking cigarettes, drinking alcohol, and illicit drug use. Prior to starting his run, his heart rate was 70/min, and the blood pressure was 114/74 mm Hg. Immediately after stopping his run, the heart rate is 130/min, and the blood pressure is 145/75 mm Hg. Which of the following is most likely responsible for the change in his heart rate and blood pressure? Options: A) Baroreceptor reflex B) Increased activation of the vagal nuclei C) Sympathetic nervous system activation D) Vasopressin release from the pituitary Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.