Which of the following is an example of a hierarchy?

A graduation ceremony, where students whose names begin with A sit in the first row, B in the second row, C in the third row, and so on through Z.
The United States government, where our executive, legislative, and judicial branches share power through a system of checks and balances.
A roller coaster line, where only people who are four feet or taller are allowed to ride.
The U.S. Army, where staff sergeants must report to their lieutenants, who must in turn report to their captains, who must report to the lieutenant colonels, and so on, through to the generals.