Ministry of Foreign Affairs, Japan, 1974
Buku Teks
Rodgers, William H.
St. Paul Minn, 1977
Buku Teks
Rodgers, William H.
West Publishing, 1994
Buku Teks
Kubasek, Nancy K.
Pearson - Prentice Hall, 2005
Buku Teks
Gaba, Jeffrey M.
United States of America, 1994
Buku Teks