Scientists recently discovered that pictures on cave walls at Creswell Crags are the oldest known in Great Britain. But they didn't find out in the usual way. Archaeologists often date cave art with a process called radiocarbon dating. The technique can measure the age of carbon found in charcoal drawings or painted pictures. Carbon is an element found in many things, including charcoal and even people. But in this case, there was no paint or charcoal to test. People carved the pictures of animals and figures into the rock using stone tools. The scientists had an "aha!" moment when they noticed small rocks stuck to the top of the drawings. The small rocks must have formed after the drawings were made. "It is rare to be able to scientifically date rock art," said Alistair Pike, an archaeological scientist at Britain's University of Bristol. "We were very fortunate that some of the engravings were covered by stalagmites . When a test proved that the stalagmites formed 12,800 years ago, the scientists knew the art underneath them had to be at least that old. And some of the animals shown, like the European bison, are now extinct--another tip-off that the art is quite old. The artists came to Creswell Crags, This place is one of the farthest points north reached by our ancient ancestors during the Ice Age. At that time, much of the North Sea was dry, so people could move about more easily. Some tools and bones found there are 13,000 to 15,000 years old. They show that the travelers hunted horses, reindeer, and arctic hare. Their artwork is similar to art in France and Germany. It tells scientists that the Creswell Crags artists must have had a close connection to peoples several thousand kilometers away-another important clue to understanding how humans spread out across the world. By studying the cave art, scientists know something about   _  .
how ancient people crossed the North Sea why some of the animals have died out how humans spread out across the world what kind of animals people hunt at that time
Question: Scientists recently discovered that pictures on cave walls at Creswell Crags are the oldest known in Great Britain. But they didn't find out in the usual way. Archaeologists often date cave art with a process called radiocarbon dating. The technique can measure the age of carbon found in charcoal drawings or painted pictures. Carbon is an element found in many things, including charcoal and even people. But in this case, there was no paint or charcoal to test. People carved the pictures of animals and figures into the rock using stone tools. The scientists had an "aha!" moment when they noticed small rocks stuck to the top of the drawings. The small rocks must have formed after the drawings were made. "It is rare to be able to scientifically date rock art," said Alistair Pike, an archaeological scientist at Britain's University of Bristol. "We were very fortunate that some of the engravings were covered by stalagmites . When a test proved that the stalagmites formed 12,800 years ago, the scientists knew the art underneath them had to be at least that old. And some of the animals shown, like the European bison, are now extinct--another tip-off that the art is quite old. The artists came to Creswell Crags, This place is one of the farthest points north reached by our ancient ancestors during the Ice Age. At that time, much of the North Sea was dry, so people could move about more easily. Some tools and bones found there are 13,000 to 15,000 years old. They show that the travelers hunted horses, reindeer, and arctic hare. Their artwork is similar to art in France and Germany. It tells scientists that the Creswell Crags artists must have had a close connection to peoples several thousand kilometers away-another important clue to understanding how humans spread out across the world. By studying the cave art, scientists know something about   _  .
A. how ancient people crossed the North Sea
B. why some of the animals have died out
C. how humans spread out across the world
D. what kind of animals people hunt at that time
Answer: