Hornby, Albert Sydney, author
Oxford University Press, 1995
Buku Referensi
[publisher not identified], [date of publication not identified]
Buku Referensi
Dale, Paulette, author
Pearson Education, 2005
Buku Teks
Hunter, Lynne, author
Prentice Hall Regents, 1989
Buku Teks
Martin Montgomery ... [and others]
Routledge , 2007
Buku Teks