Anderson is reading his morning paper and sees the following headline: "men should avoid rock music when playing board games." (this headline is based on a study conducted by fancourt, burton, & williamon, 2016.) in the study, men and women played the game "operation" when listening to different types of music. male participants performed worse when listening to ac/dc than when listening to mozart, but female participants’ performance did not differ based on music. how many variables are included in this study?