The rise of the Incan empire rested not only on the Incas' military might and the ability of their rulers but also they imposed a highly organized economic and political system on many different ethnic groups and permitting them to retain many of their customs and often their own leaders. B
(A) they imposed a highly organized economic and political system on many different ethnic groups and permitting them
(B) on their imposition of a highly organized economic and political system on many different ethnic groups, who were permitted
(C) because of their imposition of a highly organized economic and political system on many different ethnic groups, who they permitted
(D) on their imposing of a highly organized economic and political system on many different ethnic groups and they permitted them
(E) imposing a highly organized economic and political system on many different ethnic groups, permitting them
如题，B选项中，who refers to groups ?~
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