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Determine what type of observational study is described. Explain. Researchers wanted to determine whether there was an association between city driving and stomach ulcers. They selected a sample of 900 young adults and followed them for a twenty-year period. At the start of the study none of the participants was suffering from a stomach ulcer. Each person kept track of the number of hours per week they spent driving in city trafc. At the end of the study each participant undenvent tests to determine whether they were suffering from a stomach ulcer. The researchers analyzed the results to determine whether there was an association between city driving and stomach ulcers. 0 A. The observational study is a crosssectional study because information is collected at a specic point in time. 0 B. The observational study is a cohort study because individuals are observed over a long period of time. 0 C. The observational study is a retrospective study because individuals are asked to look back in time. Adrug company wanted to test a new depression medication. The researchers found 700 adults aged 25-35 and randomly assigned them to two groups. The rst group received the new drug, while the second received a placebo. After one month of treatment, the percentage of each group whose depression symptoms decreased was recorded and compared. How many levels does the treatment in this experiment have? 0 A. 1 (months of treatment) 0 B. 10 (age span of respondents) O c. 2 (medication or placebo) O D. 700 (number of respondents)