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 square rubber block is subjected to a shear strain of $\gamma $_xy=40*10^(-6)x+20*10^(-6)y, where x and y are in mm.This deformation is in the shape shown by the dashed lines, where all the lines parallel to the y axis remain vertical after the deformation. Determine the normal strain along edge BC. <image 1>
A. 4.00*10^(-3) mm/mm
B. 6.00*10^(-3) mm/mm
C. 6.00*10^(-6) mm/mm
D. 4.00*10^(-6) mm/mm

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
B
The square rubber block is subjected to a shear strain of $\gamma $_xy=40*10^(-6)x+20*10^(-6)y, where x and y are in mm.This deformation is in the shape shown by the dashed lines, where all the lines parallel to the y axis remain vertical after the deformation. Determine the normal strain along edge BC. <image 1>
A. 4.00*10^(-3) mm/mm
B. 6.00*10^(-3) mm/mm
C. 6.00*10^(-6) mm/mm
D. 4.00*10^(-6) mm/mm

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