题目选项分析

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.

In the 1820's, anthracite coalfields opened in eastern Pennsylvania, and with canals and eventually railroads reaching into the region, there had been substantial supplies of coal exported to the nation's growing mills, forges, factories, and railways.