The School Psychologist at Happy Elementary School Conducted a Study

The school psychologist at Happy Elementary School conducted a study where the students of one fourth-grade class were given five recesses during the day while students in the other class were allowed the standard two recesses. The psychologist explained to the teachers that more breaks should lead to better behavior in the classrooms and then had the teachers observe the children's progress. In this experiment, there is the risk of

A) random sampling.
B) experimenter bias.
C) participant bias.
D) the placebo effect.