A typical hour of prime-time television shows three to five violent acts. Research has shown that there is an association between time spent watching TV and aggressive behavior by adolescents. We conclude that watching TV causes aggression. 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.