Which Situation Represents a Threat to Internal Validity in an Experimental

Which situation represents a threat to internal validity in an experimental study measuring the effect of a videotape of discharge instructions for patients who underwent coronary artery bypass graft (CABG) surgery?

A) Both men and women undergoing CABG surgery were included as subjects in the study.
B) Two new surgeons began performing CABG surgery during the study period.
C) Patients in the experimental group discussed aspects of the videotape with patients in the control group.
D) Data collection for the videotape and control groups took 1 year.

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