Which of the Following Would Constitute a "Natural" Experiment

Which of the following would constitute a "natural" experiment?

A) Conducting a survey of city park workers regarding their job satisfaction
B) Doing a comparative analysis of families that choose to stay and those who choose to move out of a racially changing neighborhood to find out the underlying causes for the difference in their choices
C) Evaluating a drug courts program on drug user relapses
D) Presenting half of a classroom with a documentary on discrimination,then measuring student opinions on race
E) Soliciting teenagers at a mall to view a Pickup commercial and discuss their impressions