Answer: AB and CD are perpendicular.
Explanation: You can calculate the slopes to see if they are parallel or perpendicular.
1. If they are parallel they must have the same slope.
Calculate slope of AB, call it

Slope of CD:

As we can see the lines are no parallel because their slopes are not the same.
2. If they are perpendicular then
and they are!
Therefore, AB and CD are perpendicular.