Which of the following series is called the “formal power series”?

• A. b0+b1x+b2x2+…+bnxn
• B. b1x+b2x2+…+bnxn
• C. 1/2b0+1/3b1x+1/4b2x2+…+1/nbnxn
• D. n2(b0+b1x+b2x2+…+bnxn)