Yes, we agree that although "of" is not unheard of, the preferred preposition in this sentence should be "in" and when it was brought to our attention we added "The decrease in quality." as an accepted translation. Please see the comment just above yours which was made a year ago. Our Incubator shows that "The decrease in quality." was added 2016-12-04 07:42:58 UTC.
So if your sentence was rejected it was not due to the use of "in". When a sentence is rejected you should make a Report. Thank you for your input as all information is appreciated as a valuable addition to the course.
Here's how to report.
Go to the bottom of the exercise page where you'll see REPORT Click on that:
Then choose what you want to REPORT: (You'll see these to choose from.)
-The audio does not sound correct.
-The dictionary hints on hover are wrong or missing.
-The Greek sentence is unnatural or has an error.
-The "Correct solution" is unnatural or has an error.
[-My answer should be accepted. This is the one you should click on if your translation was not accepted