Gaya APA
Willumsen, G. (2003).
30 Years of National Geographic Specials .
Washington, D.C.:
National Geographic.
Gaya MLA
Willumsen, Gail.
"30 Years of National Geographic Specials".
Washington, D.C.:
National Geographic,
2003.
GEN - Bacaan/Seri Non-Fiksi.