GWD语法一题！

Xiaotongdi

In the 1820’s, anthracite coalfields opened in eastern Pennsylvania, and when canals and eventually railroads reached into the region, there were substantial supplies of coal that was exported to the nation’s growing mills, forges, factories, and railways.

1. when canals and eventually railroads reached into the region, there were substantial supplies of coal that was
   
2. once canals and eventually railroads reaching into the region, there were substantial supplies of coal having been
   
3. with canals and eventually railroads reaching into the region, there had been substantial supplies of coal
   
4. canals and eventually railroads reached into the region, with substantial supplies of coal being
   
5. as canals and eventually railroads reached into the region, substantial supplies of coal were
   这题答案是E，但是那个and as canals and……怎么觉得怪怪的！
   

kathyhappy

as引导了一个原因状语从句，所以后面这句是as XXX reached into the region, .... 而XXX部分是一个并列成分，canals and railroads。 哪里怪啊？

Xiaotongdi

好吧，解释完就不怪了！谢谢……

lllmmm

这里 as和 when一样