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: 兩個they不是指代同一個對象
B: correct
C: 兩個they不是指代同一個對象
D: but不能引導條件狀語從句
E: 同上
請問兩個they分別指代什麼?
我覺得好像都是代替原題主詞Industrialization and modern methods啊!