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Study Says Dogs Can Smell Cancer

Dogs are known for their sense of smell.They can find missing people and things like
bombs and illegal drugs.Now a study suggests that the animal known as man's best friend
can even find bladder（膀胱）cancer.
Cancer cells are thought to produce chemicals with unusual odors（气味）.
Researchers think dogs have the ability to smell these odors,even in very small
amounts,in urine（尿）.The sense of smell in dogs is thousands of times better than
in humans.
The study follows reports of cases where,for example,a dog showed great interest in
a growth on the leg of its owner.The mole（痣）was later found to be skin cancer.
Carolyn Willis led a team of researchers at Arnersham Hospital in England.They
trained different kinds of dogs for the experiment.The study involved urine collected from
biadder cancer patients,from people with other diseases and from healthy people.
Each dog was tested eight times.In each test there were seven samples for the dogs
to smell.The dog was supposed to signal the one from a bladder cancer patient by lying
down next to it,
Two cocker spaniels（短腿长毛垂耳小猎犬）were correct fifty-six percent of the time.
But the scientists reported an average success rate of forty-one percent.
As a group,the study found that the dogs chose the correct sample twenty-two out of fifty-four times.That is almost three times more often than would be expected by chance alone.
The British Medical Journal published the research.In all,thirty-six bladder cancer
patients and one hundred and eight other people took part.
During training,all the dogs reportedly even identified a cancer in a person who had
tested healthy before the study,Doctors found a growth on the person's right kidney(肾）.
Bladder cancer is the ninth most common cancer worldwide.The International Agency
for Research on Cancer says this disease kills more than one hundred thousand people each
year.Doctors say cigarette smoking is the leading cause of bladder cancer.

The dogs' average success rate was
A:56%.
B:41%.
C:22%.
D:54%.