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': "The commonest method of detection of diphtheria carriers is: \nSelect and explain. \nA. Schick test, B. Dick test, C. Casoni's test, D. Charles' test", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Schick test, method for determining susceptibility to diphtheria; it laid the basis for inoculation against the disease. A minute amount of diphtheria toxin is injected into the skin of the forearm.\xa0\nRedness at the site of injection after three days indicates a positive reaction (absence of circulating antibody) or a false positive reaction (hypersensitivity to the toxin).\xa0\nA positive reaction can be distinguished by use of a control injection of the same amount of heated toxin (toxoid) into the other forearm.', 'role': 'assistant'}]