You are answering a USMLE-style medical question. Question: An investigator is studying the effects of different gastrointestinal regulatory substances. A healthy subject is asked to eat a meal at hour 0, and the pH of stomach contents and rate of stomach acid secretions are measured over the next 4 hours. Results of the study are shown. Which of the following mechanisms most likely contributes to the changes seen at point D in the graph? Options: A) Increased vagal stimulation B) Increased activity of D cells C) Increased activity of enterochromaffin-like cells D) Increased activity of I cells Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.