By the definition of a perpendicular line, the angles are all 90 degrees. They are all corresponding angles, and by the corresponding angles postulate, line C is parallel to line D.

It's been a year since I did Geometry. I'm not sure if this would be the correct way to word it, but use the perpendiculars to prove that C and D are transversals of lines A and B, hereby using the Corresponding Angles Postulate. Then, by the converse, C and D are parallel.