Based on slope, how do you know when two lines are perpendicular? A. The slope of one line must be equal to reciprocal of the other for the two to be perpendicular. B. The slope of one line must be the negative reciprocal of the other for the two to be perpendicular. C. If the slopes of the 2 lines are equal, then they are perpendicular. D. The slope of one line must be the negative of the other slope for the two to be perpendicular.