Research Has Shown That Patients' Expectations of Treatment Effectiveness Are

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. Sandra, 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 1, but jeans and a t-shirt when working with experimental Group 2. This created a confounding variable. This confounding variable was a _____

A) selection bias.
B) experimenter bias.
C) placebo effect.
D) random effect.