Neather, E.J.
MacMillan Press , 1985
Buku Teks
Hutagalung, Rory Anthony, author
Gramedia Pustaka Utama, 2003, 2003
Buku Teks
Kurbegov, Eliane
Mc Graw Hill, 2012
Buku Teks
Kirk-Greene, C. W. E.
Foulsham, 1992
Buku Teks
Burriss, Eli E.
Prentice-Hall, 1958
Buku Teks