pattern - Seemingly difficult culture fair question

Select the correct choice from the bottom 6 options to fill the final box above the horizontal line.

I stumbled upon this seemingly difficult question while doing a Mensa practice test. I have since been trying to figure it out but I can't. I am calling it seemingly difficult, because these questions always appear to be difficult, until you come up or are provided with, their simple and elegant answer. I hope someone can help me!

Answer

It is

the bottom right one.

Because

In each column, flip the top image over the horizontal axis. Overlay the second image; where both images have lines, erase the line. You end up with the third image.