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': 'True regarding pertusis is all, except - \nSelect and explain. \nA. Infectivity is highest in paroxysmal state, B. Cross immunity is not seen with B. parapertusis, C. Chronic carrier state is not seen, D. Secondary attack rate is 90%', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]