Cho S = 1 + x +

Câu hỏi: Cho S = 1 + x + x2 + x3 + x4 + x5, chọn câu đúng

A. xS – S = x6 – 1 

B. xS – S = x6

C. xS – S = x6 + 1

D. xS – S = x7 – 1