Which statement best explains how Kublai Khan conquered the Sung dynasty?

A.Kublai Khan sent spies to learn about key leaders among the Sung.
B.Kublai Khan convinced many Mongol warriors to fight against the Sung.
C.Kublai Khan formed alliances with military forces in Japan and Southeast Asia.
D.Kublai Khan learned to use siege warfare and trebuchets to overcome Sung defenses.