Final answer:
At least two lines must be level when setting up a ladder: the base of the ladder and the top of the ladder.
Step-by-step explanation:
When setting up a ladder, at least two lines must be level. These lines are the base of the ladder and the top of the ladder.
It is important for both the base and the top of the ladder to be level in order to ensure stability and prevent the ladder from tipping over. If the base of the ladder is not level, it may wobble or slide, while an uneven top can cause the ladder to lean or slip sideways. By leveling both the base and the top, the ladder will be securely positioned and safe to use.