Which statement BEST explains why the model is useful for analyzing global changes to Earth systems? A. The causes and effects of global changes involve one Earth system only. B. The causes of global changes involve two Earth systems, and the effects involve the remaining two Earth systems. C. Earth systems interact with one another, and the causes and effects of global changes involve all of the systems. D. The causes and effects of global changes act separately on each Earth system, and the systems do not interact with one another.