The transcontinental railroad made it much easier and faster to bring goods and people across the continental United States. This made shipping easier and cheaper, and commerce increased drastically (Answer is B).
The railroad made many people, namely capitalists, incredibly rich, and very influential in government. Corruption naturally occurred, but it was not so huge as to have a net negative effect on commerce, since the gains brought by the railroad were so great.
The railroad did not make all trading posts disappear. Although some trade posts that were not located on or near the railroad may have become less relevant, a large number of trade posts thrived thanks to the traffic brought by the railroad.
