So I'm confused. "Không có gì" (sometimes "chi" too) can also mean "you're welcome", what's the context of the "you're welcome" and why does it the same as "nothing"? My guess is it is like "no problem" but more confirmation would be make me more confident
So when the person is not quite sure of the answer, and uses an intuitive guess, they come up with the true statement "everything is temporary" or "nothing is permanent". But they would be wrong. DuoLingo actually works against them.