Plack, Margaret M., author
Slack Incorporated, 2011
Buku Teks
Journal of Population, 6 (1-2) 2000 : 79-100, 2000
Artikel Jurnal
DeMars, Brent, author
Thompson, 2004
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People's Publshing House, [date of publication not identified]
Buku Teks
Babcock, Chester D., author
Scott, Foresman, 1956
Buku Teks