World Health Organization
WHO, 2005
Buku Teks
Johnson, Earl S., author
Macmillan, 1956
Buku Teks
The Free Press, 1972
Buku Teks
Allen, Widdowson, author
[publisher not identified], [date of publication not identified]
Buku Teks
Education Departement , 1975
Buku Teks