A medical researcher studies the correlation between the number of hours spent at office jobs and the incidence of chronic back
ache. Which statement BEST describes why the investigation is an observational study? A) The researcher does not use a control group.
B) The researcher does not intervene to vary conditions.
C) It is not possible to obtain a random sample of office workers.
D) It is not possible to establish a cause-effect relation between the variables.
