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>What are the indices for the direction represented by the vector that has been drawn within a unit cell? A negative index is indicated with a minus sign ('-') in front of (rather than over) the index number. <image 1>
A. [-321]
B. [-420]
C. [-431]
D. [-430]

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
D
What are the indices for the direction represented by the vector that has been drawn within a unit cell? A negative index is indicated with a minus sign ('-') in front of (rather than over) the index number. <image 1>
A. [-321]
B. [-420]
C. [-431]
D. [-430]

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