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: Social & Preventive Medicine
[{'content': 'The commonest vector of plague is: \nSelect and explain. \nA. Xenopsylla cheopis, B. Musca domestica, C. Xenopsylla astia, D. Pulex irritans', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Xenopsylla cheopis is the commonest and the most efficient vector of plague. Other vectors include Xenopsylla astia, Xenopsylla brasiliensis and Pulex irritans. Ref: Park 21st edition: page 268.', 'role': 'assistant'}]