Tank, Ronald W.
Oxford University Press, 1983
Buku Teks
Montgomery, Carla W., author
McGraw-Hill, 2003
Buku Teks
Reichard, James S
McGraw-Hill , 2014
Buku Teks
Strahler, Arthur N., author
John Wiley & Sons, 1973
Buku Teks
United Nations, 1993
Buku Teks