19. Industrialization and modern 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. 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 some 100,000 dangerous chemical pollutants at the same time, which have D. but introducing some 100,000 dangerous chemical pollutants at the same time that have E. but at the same time introducing some 100,000 dangerous chemical pollutants, having
A: 两个they不是指代同一个对象
B: correct C: 两个they不是指代同一个对象
D: but不能引导条件状语从句
E: 同上 注意：
1、 出现两个选项一个有代词，一个没有代词，需要比较代词是否有指代错误或重复
2、
条件状语从句的省略
When/while（时间）/if/unless（条件）/even though/even if/although（即使）/once＋名词/形容词/完整句
这是PREP语法笔记里面的解释,我不明白 D: but不能引导条件状语从句 这个解释,是我不懂某个知识点么?? 为什么说条件状语从句?
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