Artikel Jurnal
Guffey, Mary Ellen, author
Cengage Learning Asia, 2008
Buku Teks
Jaderstrom, Susan, author
McGraw-Hill Glencoe, 2003
Buku Teks
Woodhall, Victoria
Hodder & Stoughton, 1992
Buku Teks
Flower, John, author
Gramedia Pustaka Utama, 1994
Buku Teks