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     Dogs can be grouped into several categories based on the functions they provide. Pet dogsare what many people like to keep at home as companions, while working dogs are what thedisabled, police, or farmers keep to lead people, track illegal activities, or pull sleds,respectively. Nowadays, scientists have found that some dogs, known as medical detectiondogs, can help detect various diseases, including even cancers.
     Dogs have a hypersensitive detection of smell consisting of 125-300 million scent glands,as compared to about 5 million in humans. With such an exceptional ability, medical detectiondogs are able to identify different types of cancer at an early stage, which is a challenging taskfor doctors using a variety of medical devices. A recent study found that these dogs can detectlung cancer by smelling blood samples and can also detect breast, bladder, colorectal, andovarian cancers. Furthermore, medical detection dogs can be trained to warn their owners whohave life-threatening allergies before they touch any foods such as peanuts that will causeserious allergic reactions. Other scientific evidence indicates that trained dogs can identifypeople with Parkinson’s disease before they show any symptoms, therefore helping to slowdown its progression.
    Dogs can be more than just pets and companions. Working dogs are valuable partners withthe blind, law enforcement, and farmers, and now medical detection dogs are helping doctorswith the important task of helping the sick. Whether they can detect foods someone is allergicto, Parkinson’s disease, or even cancers, these dogs are proving themselves to be lifesavers.
41. What is the main idea of the passage?
(A) Dogs can learn to help the disabled.
(B) Dogs can search for missing persons.
(C) Dogs can identify diseases in humans.
(D) Dogs can do tricks to entertain people.