Một vật nhỏ dao động theo phương trình x = 4 cos ωt - π / 3 . Gọi T là chu kì dao động của vật. Pha của vật dao động tại thời điểm t = T/3 là:

A. 0 rad.

B. - π / 3 rad.

C.  2 π / 3  rad.

D.  π / 3  rad.

Read the following passage and mart the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.

The time when human 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 followers. The woolly 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 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 settled and not very numerous. It is difficult to see how they could have prevailed over the mammoth to such an extent. 

With which of the following is the passage primarily concerned?

A. Techniques used to hunt mammoths 

B. The relationship between man and mammoth in the New World 

C. The prehistory of humans 

D. Migration from Siberia to Alaska