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: Dental
[{'content': 'Humectant in tooth paste serves to \nSelect and explain. \nA. as an abrasive, B. retains moisture, C. flavoring agent, D. foaming agent', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]