Passage TwoWe all love a hero, and rescue dogs are some of the biggest heroes of all. You will often find them above and beyond duty to save someone, risking-and at times losing -their lives in the process.Rescue dogs are generally found in the Sporting and Hunting Groups, or from the traditional Herding Group. These types include the Bloodhound, Labrador retriever, Newfoundland, German Shepherd, Golden Retriever, and Belgian Malinois-all of which are chosen for search-and-rescue duty because of their amazing physical strength, loyalty, and their tendency for mental stability.These types also have a keen sense of hearing and smell-to better locate lost individuals-and are often able to access hard-to-reach areas. As highly trained animals, they serve in many different fields, including specialist search, snow slide rescue, dead body location, and tracking.To overcome obstacles and succeed when performing the demanding duties of a search-and-rescue worker, a dog must display certain qualities. In addition to intelligence and strength, the dog must be swift, confident, easily trainable, adaptable, and have a high level of stamina（耐力） and endurance. A strong sense of group cooperation and an ability to engage in friendly play during "down" time is also required of search-and-rescue dogs.A rescue dog goes through many, many hours of intensive training to be fit for duty. Training is not for the faint-hearted. Certification training can take from two to three years, working three to four hours a day, three to six days a week, often in group, team-oriented sessions.Each search-and-rescue field requires different types of training. Rescue training, for instance, includes "air scenting"--where dogs are trained to smell the air for the victim's scent（气味）and then follow the scent to the person. This ability is crucial to finding victims trapped under collapsed buildings and snow slide.What is the passage mainly about?