Numrich, Carol, author
Longman: Addison-Wesley Pub., 1995
Buku Referensi
Numrich, Carol
Longman, 1994
Buku Teks
Richards, Jack C., author
Cambridge University Press, 2015
Buku Teks
Puchta, Herbert
Cambridge University Press, 2010
Buku Teks
Herrick, Christopher, author
Longman, 2003
Buku Teks