单选题
People often say that a dog is (41) best friend. Over (42) of years, man has taught his dogs to do many kinds of work (43) guarding the home. For example, sheepdogs are famous (44) their ability to control a flock (群) of hundreds of sheep. Dogs (45) to aid disabled people for centuries. A guide dog can lead its blind owner. Nowadays, dogs can (46) to turn (47) light switches, open refrigerator doors and dial the telephone for their disabled (48) (49) the majority of people, however, dogs are simply pets and friends for (50) young and old members of the family.