Which of the following best explains mongolian khans' difficulty bringing Vietnam under the Pax Mongolica? A) The policy of defense and fortification to prepare cities for times of war
B) The policy of nonviolent resistance and diplomatic negotiation, supported by Hindu religious leaders
C) The policy of establishing strong trade ties with the Yuan dynasty to deter a conquering force
D) The policy of forced labor to quickly establish a conscripted army when facing attack