Hello, I'm Beatrix may you please help me? Which source is the most credible for researching the positive effects of the Columbian Exchange?

an article on a wiki website
a magazine published in 1988
a .gov website updated in 2019
a blog by a company that sells a product

Which leader was the first african american president at two different institutions for higher learning

Lava and other matierls luanced from a volcano and into the air are referenced as

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.