28-20: 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

为什么选Ｄ啊？Ｅ不对么，是根据逻辑意思判断的么？

谢谢了

eventually怎么放在那里？不管这个。

in the 1820's 修饰整个.....and...., 即：opened ...reached...

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 - "there were", redundant. Eliminate A
2. once canals and eventually railroads reaching into the region, there were substantial supplies of coal having been - "there were", redundant. Eliminate B
3. with canals and eventually railroads reaching into the region, there had been substantial supplies of coal  - "had been..." wrong tense. Eliminate C
4. canals and eventually railroads reached into the region, with substantial supplies of coal being - "with..." modify "canals  and eventually railroads reached...", not logical. Eliminate D
5. as canals and eventually railroads reached into the region, substantial supplies of coal were - Right! "As" here is used as conj., meaning "when".