In the 1820’s, anthracite coalfields opened in eastern Pennsylvania

雪豹Uncia

GWD TN19的一道题
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.
A.        when canals and eventually railroads reached into the region, there were substantial supplies of coal that was
B.        once canals and eventually railroads reaching into the region, there were substantial supplies of coal having been
C.        with canals and eventually railroads reaching into the region, there had been substantial supplies of coal
D.        canals and eventually railroads reached into the region, with substantial supplies of coal being
E.        as canals and eventually railroads reached into the region, substantial supplies of coal were      
正确答案是E
我想问的是标红的and 前后对应的平行内容是什么？
为什么这个and 前面还有个逗号？


按照正确答案：In the 1820’s, anthracite coalfields opened in eastern Pennsylvania, and as canals and eventually railroads reached into the region, substantial supplies of coal were exported to the nation’s growing mills, forges, factories, and railways.
是不是应该改成这样满足前后平行？
In the 1820’s, as anthracite coalfields opened in eastern Pennsylvania, and as canals and eventually railroads reached into the region, substantial supplies of coal were exported to the nation’s growing mills, forges, factories, and railways.

清酉湘竹2018

and连接的是前后2个句子
第一个句子主干： anthracite coalfields opened....
第二个句子主干：substantial supplies of coal were.....
as....是后面那个句子的前缀修饰~
学RON神用简单例子举例：
In 1990 I was born, and when I was at 5, my mom bought me a dress. 或者 In 1990 I was born, and as I reached the age of five, my mom bought me a dress.

雪豹Uncia

清酉湘竹2018 发表于 2018-10-12 11:21
and连接的是前后2个句子
第一个句子主干： anthracite coalfields opened....
第二个句子主干：substantial ...

谢谢！