This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Nuclear bag fibers arc related to: \nSelect and explain. \nA. Force, B. Length, C. Tone, D. Length and velocity', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. d. Length and velocity', 'role': 'assistant'}]