单项选择题

Dogs have been man’s best friends (41) thousands of years. Many stories are told about how brave dogs helped the people (42) trouble. With the help of the dogs, many people lost in the snow were able (43) home.
Dogs have also helped in the research work in many fields of science. The first world’s space-traveler (44) a dog named Laika. It was (45) in a man-made earth (46) in 1957 by Russian scientists.
Dogs have been film stars, (47) . A dog called Lassie was the star in quite a few American films. Dogs can hear and smell (48) than man, but they can not see so well. A dog may (49) for about 12 or 13 years. A thirteen-year-old child is not (50) grown up, but a thirteen-year-old dog is a very old dog indeed.