Which sentence uses punctuation correctly?

a.
you are going to fall off the cliff unless you move back.

b.
i wore my special hiking boots, so that i would not lose my grip.

c.
i won't leave you alone, until you retreat from the edge

The question below refers to the selection “ozymandias” by percy bysshe shelley. the traveler seems to believe that — a. monuments are eternal b. the passage of time is necessary and inevitable c. we must preserve relics of past civilizations d. even the greatest works are destroyed by time