"A spring river flows eastward ..." and "A big river flows eastward ..." These poems describing the great rivers in China show that most rivers in China flow from west to east.

The high terrain in the west and the low terrain in the east make many rivers flow eastward, connecting the east and west traffic and facilitating the connection between the coast and the inland. Options b, c and d don't meet the meaning of the question.

So choose: a.