Duckworth, M., & Gude, K. (1999). Countdown to first certificate: Student's book. Oxford: Oxford University Press.
Chicago Style aipamenaDuckworth, Michael., và Kathy Gude. Countdown to First Certificate: Student's Book. Oxford: Oxford University Press, 1999.
MLA aipamenaDuckworth, Michael., và Kathy Gude. Countdown to First Certificate: Student's Book. Oxford: Oxford University Press, 1999.
Kontuz: berrikusi erreferentzia hauek erabili aurretik.