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Buku Referensi
editors: Normand L. Hoerr [and] Arthur Osol. with the cooperation of an editorial board and 88 contributors
Blakiston Division, McGraw-Hill, 1956
Buku Referensi
Moore, W.G.
London : Penguin Books, 1984,
Buku Referensi
Strahler, Arthur N.
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Buku Teks
Petersen, James F.
Brooks/Cole Cengage Learning, 2012
Buku Teks SO