After Cao's defeat at the naval Battle of Red Cliffs in 208 AD, China was divided into three spheres of influence, with Cao Cao dominating the north, Sun Quan (182–252 AD) dominating the south, and Liu Bei (161–223 AD) dominating the west. Cao Cao died in March 220 AD. By December his son Cao Pi (187–226 AD) had Emperor Xian relinquish the throne to him and is known posthumously as Emperor Wen of Wei. This formally ended the Han dynasty and initiated an age of conflict between three states: Cao Wei, Eastern Wu, and Shu Han.
Try to answer this question if possible (otherwise reply "unanswerable"): In what month did Cao Cao die?
March