A Researcher Examined Participants' Memory Following Emotionally Shocking Events by Manipulating

A researcher examined participants' memory following emotionally shocking events by manipulating whether participants viewed a violent or a nonviolent version of a video.The video in the two conditions was identical except for two seconds in which the violence variable was manipulated.That the video was identical except for the manipulation represents the control technique of

A) balancing the individual differences across the groups of the experiment.
B) holding conditions constant in the experiment.
C) a time-order relationship between the independent and dependent variables.
D) confounding the independent variable.