London, London Longmans
Buku Referensi
Wilkes, Angela
Passport Books, [Date of publication not identified]
Buku Teks SO
Wilkes, Angela
Chicago: A Division of NTC Pub., 1987,
Buku Referensi
Wessels, Dieter
Macmillan, 1992
Buku Teks SO
Mansfield, Katherine
Century Hutchinson, 1988
Buku Teks SO