Why does the author mention that "20 large mammal genera had become extinct in North America over the previous three million years?" A. To give an example of extinction that was caused by overexploitation by humans B. To cast doubt on the idea that the extinctions at the end of the Pleistocene epoch were unique or notable C. To provide evidence for the idea that large mammal species were able to flourish in central North America after the glaciation D. To emphasize the significance of the number of extinctions at the end of the Pleistocene epoch