A study aims to evaluate the percent of adults whose right cerebral hemisphere is larger than their left hemisphere. A group of individuals aged 20 and older representative of the American adult population is asked to come to a brain-imaging facility to have their brains scanned. Each individual's scan is then analyzed to identify whether the right or left cerebral hemisphere is larger. (a) What type of study is this? Observational study: prospective Observational study: retrospective Observational study: cross-sectional Experiment: matched pairs design Experiment: randomized block design Experiment: completely randomized design (b) What is the variable of interest? American (yes, no) Gender (male, female) Larger hemisphere (left, right) Age (younger than 20, 20 and older)