To what is Francis Scott Key referring when he mentions "hirelings and slaves" in the third stanza of "The Star Spangled Banner"?

1.) He feels that the British are mercenaries who only fight for gold.

2.) He believes that the British sailors are like slaves because they live under a monarch.

3.) He refers to the fact that the British Navy was primarily comprised of African slaves.

4.) He refers to the British Navy's practice of forcibly impressing people to work on their ships.