Dange, Sindhu S.
A anta Publications, 1984
Buku Teks SO
Murphy, Gardner
Basic Books, Inc., 1953
Buku Teks SO
Pearson Longman, 2008
Buku Teks
Epstein, T. Scarlett
Manchester University Press , 1962
Buku Teks SO
Woytinsky, W.S.
Harper & Brothers, 1957
Buku Teks SO