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': 'All are true of attack rate except \nSelect and explain. \nA. It is type of prevalence rate, B. Expressed as percentage, C. Used when population is exposed to risk for a limited period of time, D. Reflects extent of epidemic', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ref:Parks 23rd edition pg 61 Attack rate (case rate) is an incidence rate. Usually expressed in percent Used only when population is exposed to risk for a limited period of time as in an epidemic. Reflects the extent of epidemic. Attack rate = (no. of new cases of a specified disease during a specified time interval / Total population at risk during the same interval) x100', 'role': 'assistant'}]