Situmeang, John B., author
The Ary Suta Center, 2017
Artikel Jurnal
Sendall, Jane, author
Oxford University press, 2014
Buku Teks
Colley, Susan Jane
Prentice-Hall, , 1998
Buku Teks
Ogden, Jane, author
Open University Press, 2004
Buku Teks
Menonde jieng, Luis, author
Minumsa, 2006
Buku Teks