According to This Map, What Likely Played a Key Role

According to this map, what likely played a key role in Japan's ability to repel the Mongols that had overrun much of China and Korea?

A) The wall located at the northern edge of the Korean peninsula slowed Mongol progress.
B) The Sea of Japan prevented the Mongols from landing on Japan's main island.
C) The Mongol force used to attack Japan appears to have been smaller than that used in Korea.
D) The Japanese islands did not share a land border with either Korea or China.