prep中的一道题

Archaeologists in Egypt have excavated a 5,000-year-old wooden hull that is the earliest surviving example of example of a “built” boat—in other words, a boat constructed out of planks fitted together—and that thus represents a major advance, in terms of boat-building technology, over the dugout logs and reed vessels of more ancient vintage.

A. together—and that thus represents

B. together—and this has represented

C. together, and it represents

D. together that was representing

E. together to represent

答案选A

who can give me explains

prep语法手册上面有解释

我想补充一点自己做题时候的方法，不知道对不对，来问问

就是前面是一个破折号补充说明，按照GMAC的习惯应该在后面补充一个破折号说明解释结束，所以答案限定在AB中。又因为时态原因排除B