Answer:
The options are
A. Environmental disasters made farming difficult.
B. Large animals were trained to assist with farmwork.
C. Most herds of animals crossed the land bridge to Asia.
D. Large mammals such as mammoths began to die out.
The answer is D. Large mammals such as mammoths began to die out .
In the past, ancient Americans were known to hunt big mammals such as mammoths for food but at a point in time this animals began to die out.
This led to some adjustments such as hunting small animals and gathering more plants to survive,
