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Several of you including Shuaib correctly identified that the lines AB and CD cross at $90$ degrees. Flora observes that the number of squares horizontally and vertically are swapped around when you draw a perpendicular line:
They are the same number of squares although the number of squares has been rotated.Alden and Jimmy from Woodstock Primary correctly point out that :
They do not have to be the same length because they can be any length as long as it's a $90$ degree angle.