The Nurse Is Conducting a Study on Heart Clients Who

The nurse is conducting a study on heart clients who are actively involved in exercise programs following a heart attack. Which threat would the nurse point out as detrimental to internal validity?

A) Some subjects run faster on the treadmill when a researcher walks through the room.
B) Including an exciting new cross-fit training program as one of the exercise options that can accommodate a few of the study subjects
C) Your fervent belief from personal experience that intense cardiovascular exercise can speed recovery and prevent further heart attacks
D) Using a variety of stationary bicycles for stress testing