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当前学科：市政工程管理与实务
题目：
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正台阶环形开挖法适用于地质条件差的地层、修建跨度不大于（ ）的隧道。
A
6m
B
8m
C
10m
D
12m
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Scientists were embroiled (使卷入) last week in an international row over genetically modified cotton (GM cotton).A study in China suggested for the first time that the crop was permanently damaging the environment and that insects were building up resistance to it.The study, by the Nanjing institute of Environmental Science, combined the laboratory and field work from four Chinese scientific institutes. The study was done over a several - year period.GM Cotton had a gene resistance to the cotton bollworm (棉铃虫) and isolated from the bacterium (细菌) named Bacillus thuringiensis (Bt), but the study found that it harms the natural parasitic (寄生的) enemies of the cotton bollworm.It also indicated that populations of pests besides the bollworm had increased in Bt cotton fields and some had replaced the bollworm as the primary pest.However, the leading GM company, the US's Monsanto, which controls more than 80 per cent of the Bt cotton grown worldwide, dismissed the research.It said that the industry has always cited GM cotton as its biggest success, because it can increase yields by up to 60 per cent and reduce the need for pesticides (杀虫剂) by 80 per cent.But, unfortunately for the industry, (80) the scientists also found that the resistance of Bt cotton to bollworm decreased significantly over time.Why scientists say no to GM cotton?A．Because the cotton was genetically modified.B．Because the cotton was found to damage the environment.C．Because the cotton harms the bollworm.D．Because Monsanto dismissed the research.