A Research has shown that patients' expectations of treatment effectiveness are affected by what a treatment provider is wearing. A white lab coat tends to increase their expectation of effectiveness. Gloria, a research assistant, had no preconceived ideas about her research project, but by chance she wore a white lab coat when working with experimental Group but jeans and a t-shirt when working with experimental Group 2. This created a confounding variable. This confounding variable was a

A. Random effect
B. Placebo effect
C. Selection effect
D. Experimenter effect