How did silver create a global exchange network?

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.

This cartoon appeared in 1916. the topic of the cartoon isa)the reason for the start of world war i.b)the success of us submarines in world war i.c)the military alliance between germany and mexico.d)the attitude in the us towards submarine warfare in world 

An refers to a specific factor that has a range of possible values