Which of the following took place at the beginning of Genghis Khan's role in the rise and expansion of the Mongol Empire?

a Genghis Khan defeated the Mongol chieftain to gain control of the empire.

b Genghis Khan united several tribes of the Mongolian Plateau.

c Genghis Khan invaded China, placing many cities under siege.

d Genghis Khan gained control of the Silk Road.