A=Sm=a(q^m-1)/(q-1)
B=S2m-Sm=a(q^2m-1)/(q-1)-a(q^m-1)/(q-1)=a(q^2m-q^m)/(q-1)=aq^m(q^m-1)/(q-1)
C=S2m-S2m=a(q^3m-1)/(q-1)-a(q^2m-1)/(q-1)=aq^2m(q^m-1)/(q-1)
显然,AC=B^2,选B
A=Sm=a(q^m-1)/(q-1)
B=S2m-Sm=a(q^2m-1)/(q-1)-a(q^m-1)/(q-1)=a(q^2m-q^m)/(q-1)=aq^m(q^m-1)/(q-1)
C=S2m-S2m=a(q^3m-1)/(q-1)-a(q^2m-1)/(q-1)=aq^2m(q^m-1)/(q-1)
显然,AC=B^2,选B