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Source = GMATPrep test #1
Industrialization and modtern methods of insect control have improved the standard of living around the globe while at the same time they have introduced some 100,000 dangerous chemical pollutants,having gone virtually unregulated since they were developed more than 50 years ago. [size=14.007px].
a. while at the same time they have introduced some 100,000 dangerous chemical pollutants, having
b. while at the same time introducing some 100,000 dangerous chemical pollutants that have
c. while they have introduced 100,000 dangerous chemical pollutants at the same time, and have
d. but introducing some 100,000 dangerous chemical pollutants at the same time that have
e. but at the same time introduce some 100,000 dangerous chemical pollutants, having
OA: B
a项是having 作状语这里错了吗?
b项while+doing 不明白为什么可以这么用,求解~!
然后c项为什么不对呢?
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