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>The many identical residents of Whoville love drinking Zlurp. Each resident has the following willingness to pay for the tasty refreshment:<image 1>The cost of producing Zlurp is $1.50, and the competitive suppliers sell it at this price. (The supply curve is horizontal.) Mayor Grinch imposes a $1 tax on Zlurp. What is the external cost?
A. 1
B. 2
C. 3
D. 4

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
C
The many identical residents of Whoville love drinking Zlurp. Each resident has the following willingness to pay for the tasty refreshment:<image 1>The cost of producing Zlurp is $1.50, and the competitive suppliers sell it at this price. (The supply curve is horizontal.) Mayor Grinch imposes a $1 tax on Zlurp. What is the external cost?
A. 1
B. 2
C. 3
D. 4

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