Powell, Derek
Harlow Longman, 1994
Buku Teks
Butler, B.C.M.
Longman Scientific & Technical , 1988
Buku Teks
Bennison, George Mills
Routledge, 2013
Buku Teks
Anderson, Franz E.
Burgess Publishing, 1979
Buku Teks
Van Duyn, Julia
Reston Publishing Company, Inc ., 1975
Buku Teks SO