Otherwise: this illustrates a declarative sentence, the use of a cardinal adjective, and the plural of the German word for "arm" (which, simply enough, is Arme).
As to why someone might make such a statement:
She: Ich hole dir einen Korb, um die Kartoffeln zu tragen.
He: Warum? Ich habe zwei Arme.
I actually love Duolingo's sentences. Whenever I make comments like this, they are done with a smiley face, so I suppose I should add that. Some people really enjoy joking around here about the crazy sentences, and the jokes create a sense of community. It doesn't mean I think the sentences are dumb or should be exchanged for more boring sentences. There was actually a discussion about this for another sentence recently. I'll be more careful to show that I really like the sentences.
So i have to say, english is not my first born language. I tried to use hands, instead of arms. Because, arms are too similar to weapons. This is also in my language, and as well in english. But Duolingo didn't accept my answer. So, is such a big difference between arms and hands? Aren't they synonyms?