GMAT
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 once canals and eventually railroads reaching into the region, there were substantial supplies of coal having been exported to the nation's growing mills, forges, factories, and railways.
题目基本信息