Referrences:
Evelina M. Viloria, et. al., Sanayang Aklat sa Araling
Panlipunan Ekonomiks, Manila, Philippines 2000, SD
Publications, Inc., pp. 47-54
Consuelo M. Imperial, et. al.,Kayamanan ( Ekonomiks ),
Manila, Philippines, Binagong Edisyon, 2010, Rex Bookstore,
pp. 54-57, 59-65
Evelina M. Viloria, et. al., Sanayang Aklat sa Araling
Panlipunan Ekonomiks, Manila, Philippines 2000, SD
Publications, Inc., pp. 47-54
Consuelo M. Imperial, et. al.,Kayamanan ( Ekonomiks ),
Manila, Philippines, Binagong Edisyon, 2010, Rex Bookstore,
pp. 54-57, 59-65