Answer: They are perpendicular lines, because their slopes are negative reciprocals (
)
Step-by-step explanation:
The missing graph is attached.
You need to remember that the slope of a line can be found with the following formula:

First, find the slope of the line AB. The steps are:
- Choose two points on the line. These can be:

You can say that:

Substituting values into the formula and evaluating, you get that the slope of this line is:

Now you must find the slope of the line CD. The steps are shown below:
- Choose two points on the line. These can be:

- You can say that:

- Now you must substitute values into the formula and evaluate, in order to find the slope. This is:

By definition, the slopes of perpendicular lines are negative reciprocals.
Therefore, since the slopes of the lines AB and CD are negative reciprocals, you can conclude that they are perpendicular.