What Accounts for the Relative Weakness of the Song Empire

What accounts for the relative weakness of the Song Empire?

A) It never succeeded in achieving the degree of centralization that had typified the Tang Empire.
B) The scholar-gentry quickly lost influence under the Song, and the bureaucracy ceased to function effectively.
C) Lack of agricultural productivity produced a general failure of the Chinese economy during the Song dynasty.
D) The military was subordinated to the civilian administrators of the scholar-gentry, leaving the dynasty vulnerable to nomadic dynasties on the frontier.