input: The following are multiple choice questions (with answers) about medical knowledge.   **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?   (A) Increased vagal stimulation   (B) Increased activity of D cells   (C) Increased activity of enterochromaffin-like cells   (D) Increased activity of I cells
output: ['(B) Increased activity of D cells <> answer', 'The following are multiple choice questions (with answers) about medical knowledge. <> instruction', '(A) Increased vagal stimulation  (B) Increased activity of D cells  (C) Increased activity of enterochromaffin-like cells  (D) Increased activity of I cells <> possible answers', '  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? <> context']