Which of the Following Constitutes a Reason for the Beginning

Which of the following constitutes a reason for the beginning of the Mongol invasions under Chinggis Khan?

A) The need to find new grazing land for their herds,which were growing too quickly for available resources
B) The desire to spread the Mongol religion of the sky god,Tengri,as a universalizing religion
C) The wish to preempt the manufacture of silk cloth in order to break the Korean monopoly
D) The interest in finding the source of the Amur River to control its waters for farming