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': 'Effect of placebo on internal validity? \nSelect and explain. \nA. Increase internal validity, B. Decreases internal validity, C. No effect, D. May increase or decrease', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]