3- A data set that consists of a sample of individuals, households, firms, cities, states, countries, or a variety of other units, taken at a given point in time, is called a(n) . a. cross-sectional data set b. longitudinal data set c. time series data set d. experimental data set 4. Econometrics is the branch of economics that . a. studies the behavior of individual economic agents in making economic decisions b. develops and uses statistical methods for estimating economic relationships c. deals with the performance, structure, behavior, and decision-making of an economy as a whole d. applies mathematical methods to represent economic theories and solve economic problems.