What is the major source of air pollution today

The separation of powers outlined in the constitution divides the federal government into equally important parts. which of the following are among those branches? (select all that apply.)

What period of history refers to the time when people did not have written records

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.