In the diagram below lines ab and cd are _.

Perpendicular
Explanation
This is because the angle between AB and CD is 90 degrees (right angle)
AB and CD are perpendicular lines
Straight lines that do not intersect and are not in the same plane are called skew lines.
Straight lines making an angle of 90° are called perpendicular lines.
Straight lines that does not touch each other and are in the same plane are called parallel lines.
In the problem,
So, these cannot be skew lines
So these lines are perpendicular lines.
Clearly the lines touches each other and are not parallel lines.
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