Which person participates mostly in the quaternary level of an economy? A. A coal miner
B. A car factory worker
C. A college professor
D. A car mechanic

Which statement is a good description of a model? a) models are a description of nature based only on one or two variables. b) models are testable ideas but they are not very to most people. c) models are a description of nature that predicts more accurately than real events. d) models are an approximiation of a real system that omits all but most essential variables in a system.