As with your previous difficulty there was an error in the database. This sentence was accidently written with "strong" which is not grammatically correct. It will take a week or so for the correction to be in your lessons and it will continue to count "stronger" as incorrect for all but the "the rope"/"a string" version. However, the software is actually not supposed to ever give/show you anything but the "the rope"/"a string" version, so something went wrong in the software. By any chance, do you have a screen cap that I can show to the programmers?