Please help as soon as possible. Read the following excerpt from "The Significance of the Frontier in American History." What is the metaphor in this excerpt?
"Behind institutions, behind constitutional forms and modifications, lie the vital forces that call these organs into life and shape them to meet changing conditions." (APEX)
A. Comparing vital forces to organs
B. Comparing people to organs
C. Comparing vital forces to modifications
D. Comparing institutions to organs