Empress Dowager Cixi ruled China during the Qing dynasty and ultimately brought dynastic rule to its end. I don't believe it's a correct clue to say Wu Zetian is the "only Empress of china".
So many where if you dive in and answer with a famous emperor from that empire it's wrong and you have to re-read question and go 'oh no, not THAT Khan etc...'. Smart question-setting!