A potentiometer wire is $100 \mathrm{cm}$ long and a constant potential difference is maintained across it. Two cells are connected in series first to support one another and then in opposite direction. The balance points are obtained at $50 \mathrm{cm}$ and $10 \mathrm{cm}$ from the positive end of the wire in the two cases. The ratio of EMF's is :

• A

  5 : 1
• B

  5 : 4
• C

  3 : 4
• Correct answer
  D

  3 : 2

Hence, the correct answer is option: 'D'.