Some researchers have noted that the adolescents who spend lot of time playing video or computer games are at greater risk for depression and for 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.