In Piliavin et al.’s (1969) 'Subway Samaritan' study, which of the following lists contains only independent variables manipulated directly by the researchers?
The race of the victim, the speed of the train, and the size of the passenger group
The victim's condition (ill vs. drunk), the race of the victim, and the model's behavior (timing and location of help)
The gender of the active helpers, the latency of help, and the number of verbal comments made by passengers
The location of the subway carriage, the age of the victim, and the time of day the trial occurred