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
这道题我选E, NN们请指教。 | 
