Copeland, Richard W., author
Macmillan, 1970
Buku Teks
O`grady, William
Cambridge University Press, 2007
Buku Teks
Kidd
Cambridge University Press, 1973
Buku Teks
Kellett, Mary, author
Paul Chapman Publishing, 2005
Buku Teks
Blaxter, Loraine, author
Open University Press , 1996
Buku Teks