In a Screw Gauge, fifth division of the circular scale coincides with the reference line when the ratchet is closed. There are $50$ divisions on the circular scale, and the main scale moves by $0.5 \mathrm{mm}$ on a complete rotation. For a particular observation the reading on the main scale is $5 \mathrm{mm}$ and the ${20}^{\text{th }}$ division of the circular scale coincides with reference line. Calculate the true reading.

• A

  $5.20 \mathrm{mm}$
• B

  $5.00 \mathrm{mm}$
• Correct answer
  C

  $5.15 \mathrm{mm}$
• D

  $5.25 \mathrm{mm}$

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