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>Hint: Please answer the question and provide the correct option letter, e.g., A, B, C, D, E, at the end.
Question: A rectangular piece of paper $A B C D$ is $5 \mathrm{~cm}$ wide and $50 \mathrm{~cm}$ long. The paper is white on one side and grey on the other. Christina folds the strip as shown so that the vertex $B$ coincides with $M$ the midpoint of the edge $C D$. Then she folds it so that the vertex $D$ coincides with $N$ the midpoint of the edge $A B$. How big is the area of the visible white part in the diagram?
<image1>
<image2>
<image3>
Choices:
(A) $50 \mathrm{~cm}^{2}$
(B) $60 \mathrm{~cm}^{2}$
(C) $62.5 \mathrm{~cm}^{2}$
(D) $100 \mathrm{~cm}^{2}$
(E) $125 \mathrm{~cm}^{2}$ 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
Hint: Please answer the question and provide the correct option letter, e.g., A, B, C, D, E, at the end.
Question: A rectangular piece of paper $A B C D$ is $5 \mathrm{~cm}$ wide and $50 \mathrm{~cm}$ long. The paper is white on one side and grey on the other. Christina folds the strip as shown so that the vertex $B$ coincides with $M$ the midpoint of the edge $C D$. Then she folds it so that the vertex $D$ coincides with $N$ the midpoint of the edge $A B$. How big is the area of the visible white part in the diagram?
<image1>
<image2>
<image3>
Choices:
(A) $50 \mathrm{~cm}^{2}$
(B) $60 \mathrm{~cm}^{2}$
(C) $62.5 \mathrm{~cm}^{2}$
(D) $100 \mathrm{~cm}^{2}$
(E) $125 \mathrm{~cm}^{2}$ 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