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 average wave velocity value of the bored pile body at a certain site is 3555.6m/s, and the low strain reflected wave dynamic test curve of a certain column is shown in Figure 10-3, corresponding to the values of time t1, t2, and t3 in the figure ,Which of the following options is the closest value to the length of the pile? ()()<image 1>
A. 10.7m
B. 21.3m
C. 24.0m
D. 48.0m

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 average wave velocity value of the bored pile body at a certain site is 3555.6m/s, and the low strain reflected wave dynamic test curve of a certain column is shown in Figure 10-3, corresponding to the values of time t1, t2, and t3 in the figure ,Which of the following options is the closest value to the length of the pile? ()()<image 1>
A. 10.7m
B. 21.3m
C. 24.0m
D. 48.0m

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