Why Is Resentful Demoralization a Threat to the Internal Validity

Why is resentful demoralization a threat to the internal validity of sport and exercise psychology research that manipulates an independent variable?

A) Participants can vary in terms of biological factors across repeated data collection.
B) An extraneous event can have a powerful yet unforeseen influence on the data.
C) Alterations to the instruments can influence score reliability across time.
D) Participant resentment can arise from not receiving the intervention.