Why did the Mongols repeatedly fail to conquer the Sung dynasty before Kublai Khan’s conquest?

A. The Sung army had trebuchets.
B. The Sung army had a blockade.
C. The Sung army had battering rams.
D. The Sung army had greater numbers.

However, the Song dynasty was difficult to conquer because of the strategic location of Xiangyang, which became a vital position for Kublai to capture and hold. The city guarded the waterways of South China because the Han River was a major tributary into the Yangtze River.

River arrow on this map is pointing

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