UNDP and ESCAP , 1990
Buku Teks
United Nations
United Nations, 1990, c1991, c1994
Buku Teks
Ramage, Janet, 1932-, author
Oxford University Press, 1983
Buku Teks
Sorensen, Harry A., author
John Wiley & Sons, 1983
Buku Teks
Departemen Pertambangan dan Energi, 1997
Buku Teks