"This is an industrial city."
Translation:Esta ciudad es industrial.
"Esta ciudad es industrial." and "Esta es una ciudad industrial." have very similar meanings, but they're not equivalent. The translation should be the latter.
I am surprised at Duo here. I have written comments on several exercises explaining to users why an answer, which varied from the sentence given in exactly the same way as this one, was not accepted. I can understand Duo beginning to accept both answers because there are only a few cases under which the difference in structure would actually result in a different meaning, but to not accept the answer or to even essentially require the sentence be structured in a different way by the building blocks provided, is just plain wrong. Because of the more flexible placement of the subject of a sentence in Spanish, identifying what the subject of the sentence (is in Spanish is a very important task for the English speaker learning Spanish. And in this case the subjects of the Spanish sentence and the English sentence are different. I am reporting it again. June 18,2018.