[听力原文]11-15The dog has been man’s best friend for thousands of years. Dogs are friendly and faithful animals. Many stories are told of the bravery and faithfulness of dogs. A dog named Barry saved 40 persons when they were lost in the snow on the Saint Bernard Pass in Switzerland about 150 years ago.Dogs are also intelligent animals, so it is easy to train them.Dogs are also helpful in scientific research. The world’s first space-traveller was a dog named Laika. Russian scientists sent it up in a man-made Earth satellite in 1957. Dogs have been film stars, too. A dog called Lassie was the star in a number of American films.Dogs can hear and smell better than men, but they cannot see so well. You may be surprised to learn that dogs are colour-blind. A dog lives on average for about 12 years. A dog aged six months compares in age with a child six years old. A thirteen-year-old child is not yet grown up, but a thirteen-year-old dog is a very old dog indeed. Why is it easy to train dogsBecause they are（）