We all hate homework
Ava, Elle, and Hannah are doing their school homework. While each of them is working on only one subject (distinct from each other's) out of mathematics, geography, and history, the following dialog takes place:
Ava says, "I'm currently not working on mathematics."
Elle replies, "I'm currently not working on history."
Hannah also claims that she is not currently working on one of the three subjects, but it is not known to us what subject that is.
After some thought Ava shouts, "I know what homework everyone is doing!"
Given that all of them speak the truth, what is the subject that Hannah says she is not currently working on?