Evidence of the Worldwide Expansion of the Savannas at the Expense

Evidence of the worldwide expansion of the savannas at the expense of the forests at the end of the Miocene is found in the:

A) increase in ¹⁴C in the earth's atmosphere at that time
B) increase in ¹³C found in soils, bones, and teeth that date to that time
C) decrease in ¹⁴C found in soils, bones, and teeth that date to that time
D) decrease in ¹³C found in soils, bones, and teeth that date to that time