17、
5. In relation to ulcers, experts study twins in order to examine the roles of genetic factors and environmental factors.
A. right B. wrong C not mentioned
6. “Environmental effects” mentioned in the fifth paragraph refers to a clean environment with no smoke and dust surrounding the living area.
A. right B. wrong C not mentioned
7. The passage argues that ulcers are related to lifestyle.
A. right B. wrong C not mentioned
标准答案： A,B,A
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