100 mL of aqueous solution of $0.05 \mathrm{M} \mathrm{Cu}^{2+}$ is added to 1 L of 0.1 M KI solution. The resultant solution was titrated with $0.01 \mathrm{M} \mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$ solution using starch indicator till blue color disappeared. What is the volume (in mL ) of $\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$ used?

• A

  $2000$
• B

  $1000$
• Correct answer
  C

  $500$
• D

  $250$

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