The Xiongnu Threat to the Han Empire Finally Came to an End

The Xiongnu threat to the Han empire finally came to an end because

A) Of the negotiation of the Han diplomat Zhang Qian.
B) Weak Xiongnu kings were unable to raise an army through alliances with other kings.
C) Changing climatic conditions weakened the Xiongnu horses, raised on the Mongolian steppes.
D) Cultural assimilation made the Xiongnu into urban dwellers.
E) The Xiongnu confederacy broke apart into warring groups.