In a Study of Emotional Intelligence, Participants Were Asked to View

In a study of emotional intelligence, participants were asked to view the eyes of several photographs of people's faces, and then rate how likely it was that they would want to be friends with the person they are currently viewing. The pictures that participants were shown exhibited 50 facial expressions, half positively valenced and half negatively valenced, of which only the eyes of those faces were visible (the rest of the face was obscured) . Researchers found that participants with a higher emotional intelligence were more likely to befriend only those pictures of people with positive facial expressions (e.g., smiling) , whereas people with low emotional intelligence were just as likely to want to befriend pictures of positive and negative facial expressions (e.g., smiling; grimacing) . Varying the facial expressions of the photographs is most likely what type of experimental manipulation?

A) social
B) environmental
C) stimulus
D) priming