This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'About herd immunity, all are true except \nSelect and explain. \nA. Depends on clinical and subclinical cases, B. Influenced by immunization, C. Depends on the presence of alternative host, D. Herd immunity is constant', 'role': 'user'}, {'content': "Answer: D \nExplanation: Elements which contribute to herd immunity are occurrence of clinical and subclinical infection in the herd immunization of the herd herd structure Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 110", 'role': 'assistant'}]