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': 'Who discovered polymerase chain reaction \nSelect and explain. \nA. Walter Gilbert, B. Barbara Mcclintock, C. Frederick Sanger, D. Karry B mullis', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Karry B mullis discovered PCR.', 'role': 'assistant'}]