In the 20th century, Greek composers have had a significant impact on the development of avant garde and modern classical music, with figures such as Iannis Xenakis, Nikos Skalkottas, and Dimitri Mitropoulos achieving international prominence. At the same time, composers and musicians such as Mikis Theodorakis, Manos Hatzidakis, Eleni Karaindrou, Vangelis and Demis Roussos garnered an international following for their music, which include famous film scores such as Zorba the Greek, Serpico, Never on Sunday, America America, Eternity and a Day, Chariots of Fire, Blade Runner, among others. Greek American composers known for their film scores include Yanni and Basil Poledouris. Notable Greek opera singers and classical musicians of the 20th and 21st century include Maria Callas, Nana Mouskouri, Mario Frangoulis, Leonidas Kavakos, Dimitris Sgouros and others.

Answer this question, if possible (if impossible, reply "unanswerable"): Who is one of the notable Greek opera singers in the 20th century?
Maria Callas