Some researchers have noted that adolescents who spend a lot of time playing video or computer games are at greater risk of depression and violence. This is an example of
A. a valid conclusion since more time yields more aggression is a positive association.
B. an observational study with lurking variables that may explain the association.
C. a single-blind experiment because the subjects knew they were watching television.
D. a paired data experiment since we are studying both aggression and TV watching.