The Nurse Is Conducting a Study That Will Consider How

The nurse is conducting a study that will consider how heart clients respond to various interventions following a heart attack. The nurse is concerned about the threat that testing could pose to the internal validity of the study. Which factor is the nurse most concerned about?

A) The long commute that a subject must take to a testing center as part of the study might change the subject's response to an intervention.
B) One subject develops an unrelated anemia, which causes fatigue, which in turn affects the outcome of the study.
C) Multiple measurements of participants' cortisol levels could cause stress in the participants and thus influence their cortisol levels, thereby altering the outcome of the study.
D) Mistakes are made when a number of subjects who should have been assigned to one intervention group are assigned to another, which may affect the outcome of the study.