This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
[{'content': 'Toxin acting on cGMP - \nSelect and explain. \nA. Heat stable E. coli toxin, B. Heat labile E. coli toxin, C. Cholera toxin, D. Shiga toxin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]