Wilkes, Angela
Chicago: A Division of NTC Pub., 1987,
Buku Referensi
Coggle, Paul
Teach Youeself Books, 1991
Buku Teks SO
Bladon, Rachel
Passport Books, [Date of publication not identified]
Buku Teks SO
McKeane, Alasdair
Longman, 1994
Buku Teks SO
Ewing, J.H.
Student Outline Co. , 1960
Buku Teks