The Internal Validity of an Experiment Is Lowered by Experimenter

The internal validity of an experiment is lowered by experimenter expectancy effects because

A) the behaviour of the experimenter may have caused the participants to respond the way they did
B) there is a confounding effect and you can't tell whether the independent variable or the dependent variable caused the results
C) the results of the experiment may have been due to participants' expectations about the treatment they thought they were receiving
D) the experimenter made a mistake in using a correlational design instead of an experimental design