You are a helpful assistant. When the user asks a question, your response must include two parts: first, the reasoning process enclosed in <think>...</think> tags, then the final answer enclosed in <answer>...</answer> tags.Please provide a clear, concise response within <answer> </answer> tags that directly addresses the question.
system
<image>In order to assess the preventive and immunological effects of the newly developed hepatitis A vaccine, 7-year-old susceptible children in a village were immunised with the hepatitis A vaccine. After 3 years of follow-up observation, the results were as follows.<image 1> The main outcome of the study was
A. Incidence of hepatitis A
B. Prevalence of hepatitis A
C. Secondary attack rate of hepatitis A
D. Survival rate of hepatitis A

Please answer the question with option letter from given choices in the format of Answer: <option letter>. You FIRST think about the reasoning process as an internal monologue and then provide the final answer. The reasoning process MUST BE enclosed within <think> </think> tags. Please answer the final answer within the <answer> </answer> tags.
user
A
In order to assess the preventive and immunological effects of the newly developed hepatitis A vaccine, 7-year-old susceptible children in a village were immunised with the hepatitis A vaccine. After 3 years of follow-up observation, the results were as follows.<image 1> The main outcome of the study was
A. Incidence of hepatitis A
B. Prevalence of hepatitis A
C. Secondary attack rate of hepatitis A
D. Survival rate of hepatitis A

Please answer the question with option letter from given choices in the format of Answer: <option letter>. You FIRST think about the reasoning process as an internal monologue and then provide the final answer. The reasoning process MUST BE enclosed within <think> </think> tags. Please answer the final answer within the <answer> </answer> tags.
val