A study of college freshmen found that students who watched TV for more than one hour each night gained 15 pounds in the first semester, compared to those who watched less than one hour. They concluded that there is an association between watching TV and weight gain in college freshmen. Which study design did they most likely use?
A. They most likely used an experiment with students randomly assigned to watch more than one hour or less than one hour of TV each night.
B. They most likely used an experiment with students randomly assigned to groups based on how much weight they gained.
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C. They most likely used an observational study asking students to keep track of their TV habits and weight.
D. They most likely used an observational study asking students to keep track of their workout regimen and weight.