Most people think zoos are safe for animals, where struggles such as difficulty in finding food and avoiding natural enemies don't exist. Without such problems, animals in zoos should live to an 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. They develop diseases and behaviour changes. Sometimes, they even become infertile, or unable to have babies. To learn more about how elephants are affected, a team of international scientists compared the life spans of elephants born in zoos with those living outdoors in their native lands. Zoos keep detailed records of all the animals, such as birth dates, illnesses, weight and death. These records made it possible for the researchers to analyse 40 years of data on 800 African and Asian elephants in zoos across Europe. The team found that female African elephants born in zoos lived an average of 16.9 years while those in the wild lived an average of 56 years. Scientists don't yet know why wild elephants seem to be so much better than their zooraised elephants. Georgia Mason, a biologist who led the study, thinks stress and obesity   may be to blame. Zoo elephants don't get the same kind of exercise they would in the wild, and most are fat. Another finding from the study showed that Asian elephants born in zoos were more likely to die early than Asian elephants caught in the wild and brought to zoos. Mason suggests stress in the mothers in zoos might cause them to have babies that are less likely to survive. In order to let the elephants in the zoos live longer, people should   _  .
give them a good diet keep their living place clean and tidy get them to exercise as in the wild try to keep as quiet as possible