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
Effect of placebo on internal validity? 
Select and explain. 
A. Increase internal validity, B. Decreases internal validity, C. No effect, D. May increase or decrease
B: Decreases internal validity
Answer: B: Decreases internal validity 
Explanation: None