(Sorry, I changed the category of the question to Voc. and idioms)

Sorry, I changed the category of the question to Voc. and idioms

Read the following and choose the number of word which is not proper in context.

(originally it has diagram, but I couldn't post picture last time. I'll try to post again. but even if I fail, please try the following and give me your precious opinions)

In order to make a paper plane, you should leave a guide crease. This can be accomplished by ? laying the paper in portrait position and folding the left part of the paper over to the right so that it ? overlaps the other side completely. Then, fold the top left corner of the page so that it touches the crease in the ? middle and vice versa for the right side. ? Crease the paper plane back to the position where you had left the guide marks and the marks should put wings on the paper plane, the most vital part of this procedure. Still in portrait position, ? unfold the non-pointed bit of the paper plane so that the wings are not internally in the paper plane but externally. Now you have a beautiful paper plane.

The answer is #5, and we should change the word 'unfold' to 'fold.'

The original paragraph of the above is clipped from the Wikipedia Dictionary as follows.

(please, read patiently the following from Wikipedia.)

------------------

Instructions for folding paper plane

The person who is folding the piece of paper in these instructions is referred to as "the folder".


1. The folder should leave a guide crease. This can be accomplished by laying the paper in portrait position and folding the left part of the paper over to the right so that it overlaps the other side completely. Crease the fold by running your thumb over the fold. This will make your fold permanent.


2. The folder then should uncrease the sides again and fold the top left corner of the page so that it touches the crease in the middle, and vice versa for the right side.


3. The folder should crease the paper plane back to the position where the folder had left the guide marks, then they should put wings on the paper plane, the most vital part of this procedure.


4. Still in portrait position the person should fold the non-pointed bit of the paper plane (the bottom part) so it creases over the guide part but inverted outwards so that the wings are not internally in the paper plane but externally.



We can see the test paragraph is adapted from Wikipedia.

Now, my questions is..



1. In Wikipedia instruction #3, the pronoun "they" looks like 'the folder' to me.

If my suggestion is right, how 'they (plural)' represents 'the folder(single)'?

Is it possible?



2. If 'they' does not represent 'the folder', does 'they' mean 'the (guide) marks' as on the test paragraph?



Thanks.