Dr. Bloedorn is a health psychologist who researches nutrition. She is curious as to whether a new drink additive will help people consume fewer calories during a meal. The drink additive is a white, odorless, tasteless powder that a person can add to any drink. She collects a random sample of 63 overweight students on campus and measures the calories they eat during lunch using a calorimeter. She then gives this additive to the same 63 participants to use at dinner and measures how many calories they eat (again using the calorimeter). The addition of a control group that does not use the drink additive would help Dr. Bloedorn address which of the following threats to internal validity will Dr. Bloedorn be worried about?
A. History
B. Attrition
C. Instrumentation
D. Placebo effects