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>Write the signal f(t) shown in <image 1> using the step functions.
A. 4t^2 [u(t) - u(t - 5)] + 20[u(t - 2) - u(t - 5)] + 15(t - 7)[u(t - 5) - u(t - 7)]
B. 4t^2 [u(t) - u(t - 2)] + 20[u(t - 2) - u(t - 5)] + 15(t - 7)[u(t - 5) - u(t - 7)]
C. 5t^2 [u(t) - u(t - 2)] + 20[u(t - 2) - u(t - 5)] + 15(t - 7)[u(t - 5) - u(t - 7)]
D. 5t^2 [u(t) - u(t - 2)] + 20[u(t - 2) - u(t - 3)] + 15(t - 7)[u(t - 5) - u(t - 7)]

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
Write the signal f(t) shown in <image 1> using the step functions.
A. 4t^2 [u(t) - u(t - 5)] + 20[u(t - 2) - u(t - 5)] + 15(t - 7)[u(t - 5) - u(t - 7)]
B. 4t^2 [u(t) - u(t - 2)] + 20[u(t - 2) - u(t - 5)] + 15(t - 7)[u(t - 5) - u(t - 7)]
C. 5t^2 [u(t) - u(t - 2)] + 20[u(t - 2) - u(t - 5)] + 15(t - 7)[u(t - 5) - u(t - 7)]
D. 5t^2 [u(t) - u(t - 2)] + 20[u(t - 2) - u(t - 3)] + 15(t - 7)[u(t - 5) - u(t - 7)]

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