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: Pharmacology
[{'content': 'Which of the following drug has been approved for the treatment of cervical dystonia: \nSelect and explain. \nA. Botulinum toxin A., B. Onabotulinum toxin A., C. Botulinum toxin B., D. All of the above.', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]