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
[spoiler]Answer:A/E[/spoiler]