Translation:The small, green people do not climb down from the curtains? This is impossible!
The computer wouldn't accept "Aren't the small green people climbing down the curtains? This is impossible!" The answer that it requires is clunky English. The question is presented as a statement with, presumably, rising intonation in the speech to tell the listener that it's a question. A much better way of phrasing the question in English is to begin with "Aren't......." What has been given as the ideal answer would sound much better without the question mark. This needs altering because, as it stands, it remains clunky English.
Not clunky - formal. And if the course accepts contractions the number of answers doubles. By sticking to formal English there is less chance of having missing translations because people learn to stick to the formal. If they start allowing contractions here then they must in every sentence - thousands of extra translations need to be added back. I'd rather they did that with translations that are really needed.