Why was economic expansion during the Chinese Tang and Song dynasties linked to military expansion?
a. Military expansion into Vietnam introduced the Chinese to faster-growing rice, which made it possible to rice crops a year in southern China
b. Military expansion into Manchuria and Korea made it possible for the Chinese to harness new energy sources in the water buffalo and the ox
c. Military expansion into Korea and Japan introduced a new strain of drought-resistant wheat into northern China
d. Military expansion into Vietnam introduced terraced farming to China, which greatly increased the amount of land under cultivation in mountainous southern China