What was an effect of the Mongol invasions during the Kamakura shogunate? A. The invasions were costly to defend against, and they created financial problems. B. The invasions strengthened the economy because people spent money to prepare for war. C. The Mongols won and oversaw a new period of peace that lasted nearly three hundred years. D. The Mongols overthrew the shogunate and brought an end to the feudal system.