病害发生三要素（病害三角）是指：（）、（）、（）。
题型：填空题
问题：
病害发生三要素（病害三角）是指：（）、（）、（）。
题型：填空题
阅读理解。
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 zoo-raised 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 .
1. Most people think that animals in the zoos ___ .
A. have difficulty in finding food
B. often struggle with each other
C. live longer than those in the wild
D. will often fight with their enemies
2. What does the underlined word " infertile " mean ?
A. Without home to stay at .
B. Without ability to give birth .
C. Without friends to talk with .
D. Without anyone to turn to .
3. The scientists carried out a research so that they could know .
A. how long the elephants live
B. how the elephants are affected
C. how the elephants can be fed
D. how the elephants should be kept
4. In order to let the elephants in the zoos live longer , people should 
.
A. give them a good diet
B. keep their living place clean and tidy
C. get them to exercise as in the wild
D. try to keep as quiet as possible
5. Why are elephants born in the zoo likely to die early ?
A. Because they eat too much and are overweight
.
B. Because they feel upset all the time .
C. Because of their mothers' stress .
D. Because their mothers are hungry all the time .