The time when humans crossed the Arctic land bridge from Siberia to Alaska seems remote to

us today, but actually represents a late stage in the prehistory of humans, an era when polished stone implements and bows and arrows were already being used, and dogs had already been domesticated.

When these early migrants arrived in North America, they found the woods and plains dominated by three types of American mammoths. These elephants were distinguished from today's elephants mainly by their thick, shaggy coats and their huge, upward-curving tusks. They had arrived on the continent hundreds of thousands of years before their human followers. The wooly mammoth in the North, the Columbian mammoth in middle North America, and the imperial mammoth of the South, together with their distant cousins the mastodons, dominated the land. Here, as in the Old World, there is evidence that humans hunted these elephants, as shown by the numerous spear points found with mammoth remains.

Then, at the end of the Ice Age, when the last glaciers had retreated, there was a relatively sudden and widespread extinction of elephants. In the New World, both mammoths and mastodons disappeared. In the Old World, only Indian and African elephants survived.

Why did the huge, seemingly successful mammoths disappear？ Were humans connected with their extinction.* Perhaps, but at that time, although they were cunning hunters, humans were still widely scattered and not very numerous. It is difficult to see how they could have prevailed over the mammoth to such an extent.

Which of the following conclusions about mammoths does the passage support？

A．Humans hunted them to extinction.

B．The freezing temperatures of the Ice Age destroyed their food supply.

C．The cause of their extinction is not definitely known.

D．Competition with mastodons caused them to become extinct.