APA
Helgren D. M., Sager R. J., . (1995). World geography today. Austin: Holt, Rinehart and Winston.
Chicago
Helgren David M, Sager Robert J, . 1995. World geography today. Austin: Holt, Rinehart and Winston.
Harvard
Helgren D. M., Sager R. J., . (1995). World geography today. Austin: Holt, Rinehart and Winston.
MLA
Helgren David M, Sager Robert J, . World geography today. Austin: Holt, Rinehart and Winston. 1995.