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>A machine frame is made of steel having Sy = 400 MPa and Ssy = 250 MPa. When loaded in a test fixture, the stresses were found to vary linearly with load. Two points on the surface were found to be most critical. With a 4-kN test load, stresses at these points were: point a, $\sigma $1 = 200 MPa,$\sigma $2 = 100 MPa; point b, $\sigma $1 = 150 MPa, $\sigma $2 = -100 MPa. Compute the test load at which the frame will experience initial yielding according to the maximum-normal-stress theory.<image 1>
A. 5kN
B. 6kN
C. 7kN
D. 8kN

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
D
A machine frame is made of steel having Sy = 400 MPa and Ssy = 250 MPa. When loaded in a test fixture, the stresses were found to vary linearly with load. Two points on the surface were found to be most critical. With a 4-kN test load, stresses at these points were: point a, $\sigma $1 = 200 MPa,$\sigma $2 = 100 MPa; point b, $\sigma $1 = 150 MPa, $\sigma $2 = -100 MPa. Compute the test load at which the frame will experience initial yielding according to the maximum-normal-stress theory.<image 1>
A. 5kN
B. 6kN
C. 7kN
D. 8kN

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