Answer:
D. Large mammals such as mammoths began to die out.
Explanation:
The ancient north Americans were said to have lived between 40,000 to 10,000 years ago, this is the same period when the Mammoth (specifically Woolly Mammoth), a variation of elephant species lived in the region. However, as a result of climatic changes, all these mammoth species died out in the region.
Hence, in this case, the factor that caused ancient Americans to hunt small animals and gather more plants to survive is "Large mammals such as mammoths began to die out."
