Answer: Choice A
A diagram is shown below.
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Step-by-step explanation:
First we make a copy of AB. Then we tack on a copy of CD onto AB. Check out the diagram below to see what I mean. The segments AB and CD glue together to form the larger segment AD. We can say AB+CD = AD. Dividing that larger segment in half will locate the midpoint and it will help us average the lengths of AB and CD. It's the visual or geometric meaning of what "average" means.
To find the midpoint of any segment, we involve the process of finding the perpendicular bisector. Recall that "bisect" means to "cut in half". The perpendicular bisector of XM is point Y, and XY is exactly half that of segment AD.
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Check out the diagram below.
Figure 1 is the two segments separated, and figure 2 has us glue them together to form segment AD. Segment AD is renamed to XZ. Finding the perpendicular bisector of XZ leads to point Y at the midpoint and
, which is shown in figure 3.