Dr. Milano compares the performance of a group of 20-year-olds, a group of 45-year-olds, and a group of 70-year-olds on several tests of fluid intelligence and several tests of crystallized intelligence. Dr. Milano's investigation BEST exemplifies a:

cross-sectional study          

Explanation:

Dr. Milano compares the performance of a group of 20-year-olds, a group of 45-year-olds, and a group of 70-year-olds on several tests of fluid intelligence and several tests of crystallized intelligence. Dr. Milano's investigation BEST exemplifies a cross-sectional study. A cross-sectional study is a type of observational study that analyzes data from a population, or a representative subset, at a specific point in time, it involves the use of different set of people who differ in the trait of interest. For example, the effect of age on intelligence level will involve the selection of subjects across the cross-section of ages of interest.

the answer is b, encourage free trade