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>As shown in the diagram, vertical leveling staffs are set up at points A and B. The level instrument is placed at point M. The reading on the A staff at the center crosshair is $a'_{M}=1.355m$, and the reading on the B staff at the center crosshair is $b'_{M}=1.455m$. When the level instrument is moved to point N, the reading on the A staff is $a'_{N}=1.585m$, and the reading on the B staff is $b'_{N}=1.485m$. What is the correct height difference $h_{AB}$ between points A and B?<image 1>
A. -0.01m
B. +0.02m
C. 0
D. +0.01m

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
As shown in the diagram, vertical leveling staffs are set up at points A and B. The level instrument is placed at point M. The reading on the A staff at the center crosshair is $a'_{M}=1.355m$, and the reading on the B staff at the center crosshair is $b'_{M}=1.455m$. When the level instrument is moved to point N, the reading on the A staff is $a'_{N}=1.585m$, and the reading on the B staff is $b'_{N}=1.485m$. What is the correct height difference $h_{AB}$ between points A and B?<image 1>
A. -0.01m
B. +0.02m
C. 0
D. +0.01m

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