Which of These Situations Describes a Reactive (Hawthorne)effect

Which of these situations describes a reactive (Hawthorne) effect?

A) Control participants in an intervention study to reduce anxiety also experience a reduction in anxiety.
B) The participants in an intervention support group continue the support group after the study has ended.
C) Participants become concerned after learning that they received a placebo, not the treatment, during the study.
D) Study participants want to know the outcomes of the study after it is completed.