The Nurse Analyzes Published Research Studies on the Number of Heart

The nurse analyzes published research studies on the number of heart clients who are actively involved in exercise programs following a heart attack in preparation of conducting similar research. Which threat to external validity must be considered as the nurse prepares to conduct this research?

A) Loss of a subject due to a second heart attack
B) Selection of a subject who does not meet the eligibility criteria for the study
C) A subject pedaling faster on the stationary bicycle whenever an examiner walks in the room
D) An unexpected increase in a subject's appetite that could affect the study