The width of the box to drop the words in is determined by the correct answer possibilities. If one of the distractors (in the extra option 'distractors') is longer than any of the correct answers then this won't fit into the box, so the user knows it can not be right.
The width of the box is determined by the distractors in the Gap Fill Text though.
So the work-around is to put the lengthy distractors there as well.
