This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Physiology
[{'content': 'which of the following is true regarding nuclear bag fibres of motorunit ? \nSelect and explain. \nA. responds to length only, B. responds to both length and velocity changes, C. responds to contraction of muscle fibres, D. responds to reflex relaxation', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Two types of intrafusal fibres -- nuclear bag fibre * primary (la) afferent -- supplies annulospiral ending in the centre -- provide information on muscle length and velocity (phasic response) fast stretch reflex -- nuclear chain fibre * primary (la) and secondary (II) afferent -- supplies flower spray ending -- monitor the length of the muscle (tonic response) --slow stretch reflex ref : guyton and hall', 'role': 'assistant'}]