Why did the creators of this country create this three level system

Ineed answers 5 and 6 on my social studies homework reply to this question i posted

Which of the following phrases best describes american society in the early 1900s

How did inca develop their road system? a) they used mortar to hold the stones together. b) they built suspension bridges over rivers. c) they mixed copper and stone together to make the blocks. d) they laid roads in the cities, not in the less densely populated deserts.