The second phase of humanity began: A. As nations developed strong militaries.
B. The phase of hunter-gatherers in Africa;
C. With the development of agricultural centers;
D. The development of great empires;

What is the main difference between primary and secondary sources? a. primary sources are historical documents, and secondary sources are modern-day documents. b. primary sources are derived from museums and archives, and secondary sources are created by history students and academics. c. primary sources are found in ancient places with a rich history, and secondary sources are found in historical journals and blogs. d. primary sources are created during a particular historical era, and secondary sources are created by studying primary sources.