Why Do Researchers in the Area of Mental Imagery Need

Why do researchers in the area of mental imagery need to be concerned about experimenter expectancy?

A) Experimenters might somehow convey their hypotheses to the participants.
B) Experimenters typically expect participants to perform very well on imagery tasks.
C) When experimenters expect a certain outcome in their research,the participants unconsciously produce results that contradict this expectancy.
D) Experimenter expectancy typically increases the magnitude of individual differences.