Choose from

1. Two lines are perpendicular if the dot product of their direction vectors is zero.

2. Two lines are perpendicular if the product of their (non-zero) slopes is -1. If one has a zero slope, the slope of the other must be "undefined" and vice versa.

3. Two lines are perpendicular if the angle between them is 90 degrees.

1. Two lines are perpendicular if the dot product of their direction vectors is zero.

2. Two lines are perpendicular if the product of their (non-zero) slopes is -1. If one has a zero slope, the slope of the other must be "undefined" and vice versa.

3. Two lines are perpendicular if the angle between them is 90 degrees.