After a lengthy research on meinetwegen, meinethalben and the likes, I honestly believe it should be deswegen. Deshalb must had been used for something else, but that meaning is gone and it's now accepted as an equal of deswegen. If you care to be "nice" to the language, please use deswegen.
Duolingo teaches American English, and uses American English to teach other languages.
(With a handful of exceptions; for example, the Welsh course uses mostly British English as its teaching language.)
So if leftpondian usage grates on you, perhaps Duolingo is not the place for you.
In German, the conjugated verb must always be second in the sentence, so if you place something other than the subject in first place, the subject ends up in third place, rather than second.
Or if your brain starts swimming at the mere mention of a predicate nominative, then see here: http://yourdailygerman.com/learn-german-online-course/ (The word order link can be found under "The Box Model")
For one, sauer is a German noun which means "sour" (as in taste) or "angry". See dict.cc or Wiktionary, whereas bose is an English verb which describes a technigue, usually associated with archaeology, that is similar to echolocation (See Wiktionary).
But I presume that what you meant was "böse" (there is no German word "bose"), which means "evil" or "malicious".
Eine Nonne (a nun) might be sauer that her habit was soiled or torn. One would not expect, however, for her to be böse. Wenn sie wäre, die Priorin würde sauer sein.