New York : Science Press
Majalah, Jurnal, Buletin
Wiles, David, author
Cambridge University Press, 2011
Buku Teks
Peacock, Ronald
Harcourt, Brace and Company, Inc, 1946
Buku Klasik
England, Alan
Macmillan Education, 1990
Buku Teks
edited by Julian Hilton
Macmillan, 1993
Buku Teks