题目内容：
根据以下材料，回答题Longer Lives for Wild Elephants
Most people think of zoos as safe places for animals, where struggles such as having difficulty finding food and avoiding predators (猛兽) don't exist. Without such problems, animals in zoos should live to a ripe (成熟的) old age.
But that may not be true for the largest land animals on Earth. Scientists have known that elephants in zoos often suffer from poor health. Sometimes, they even become unable to have babies.
To learn more about how captivity (圈养) affects elephants, a team of international scientists compared the life spans of female elephants born in zoos with female elephants living outdoors in their native lands. Zoos keep detailed records of all the animals in their care, documenting factors such as birth dates, illnesses, weight and death. These records made it possible for the researchers to analyze 40 years of data on 800 African and Asian elephants in zoos across Europe. The scientists compared the life spans of the zoo-born female elephants with the life spans of thousands of wild female elephants in Africa and Asian elephants that work in logging camps (伐木场), over approximately the same time period.
The team found that female African elephants born in zoos lived an average of 16.9 years.
Their wild counterparts who died of natural causes lived an average of 56 years--more than three times as long. Female Asian elephants followed a similar pattern. In zoos, they lived 18.9 years,while those in the logging camps lived 41.7 years.
Scientists don't know yet why wild elephants seem to get on so much better than their zoo-raised counterparts. Georgia Mason, a biologist at the University of Guelph in Canada who led the study,thinks stress and obesity (肥胖症) may be to blame. Zoo elephants don't get the same kind of exercise they would in the wild, and most are very fat. Social lives of elephants are also much different in zoos than in the wild, where they live in large herds and family groups.
The study raises some questions about acquiring more elephants to keep in zoos. While some threatened and endangered species living in zoos reproduce (生殖) successfully and maintain healthy populations, that doesn't appear to be the case with elephants.
Unlike other animals in zoos, zoo-raised elephants __________ . A．live a long life.
B．give birth to many babies.
C．develop poor health.
D．have difficulty getting food.
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