Bullying Appears to Peak During the Sixth Grade in American

Bullying appears to peak during the sixth grade in American schools because:

A) this is when interest in the opposite sex is starting to rise.
B) children at this age have fewer parental controls than in earlier years.
C) children are moving from elementary to middle school, and new social groups are forming.
D) children are frustrated by the increased academic demands of middle school, and bullying is a way to "blow off steam."