PREP 1-97

melissa_gao

97. 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

Answer: B

为啥是introducing呢？？

AlienX

主從主語相同+主從謂語動作發生時態一致=>while從句主語省略, (主動)謂語變現在分詞