求OG13第10题官解里的问题

headhunter

10. The widely accepted big bang theory holds that the universe began in an explosive instant ten to twenty billion years ago and has been expanding ever since.

B 选项Had begun is the wrong tense because it describes action that occurred farther in the past than some other, specified past action.

官解：Had begun is the wrong tense because it describes action that occurred farther in the past than some other, specified past action.

小生又不才，不明白时态分类，望大牛们解释~

xiaohaotoutou

这是一个一般过去时态，表述的是一件事情，如果用HAD DONE 在GMAT 中

是为了区分 过去时态中两个动作发生的先后问题，使文章更加清晰。

xiaohaotoutou

OG 13 中有一题 是 reduced the amount of dioxides(貌似是这个) that xxx are allwoed to dump
                      reduced the amount of dioxides that xxx had been allowed to dump.

自己理解下~