请教时间状语while的省略问题

请教大家，我对时间状语从句省略结构不是很清楚，特别是涉及到while的用法。以下举了两例请大家帮忙分析一下。谢谢！！！

PREP 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

ANS: B

大全95题：  Although some officials noted that using machines for farming in China costs more than traditional hand cultivation, the mechanization of farming in the village of Long Bow doubled the corn yield while the previous year’s costs were cut in half.

(A) mechanization of farming in the village of Long Bow doubled the corn yield while the previous year’s costs were cut in half

(B) mechanization of farming in the village of Long Bow doubled the corn yield while cutting costs to half those of the previous year

ANS:B

置顶贴里有详细的解答，

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=81945&page=1