The length of an object is measured using a vernier system whose main scale is $30 \mathrm{cm}$ long with $600$ divisions. If $19$ divisions of the main scale coincide with $20$ divisions of the vernier scale, then its least count is

• A

  $0.25 \mathrm{c}\mathrm{m}$
• B

  $0.025 \mathrm{c}\mathrm{m}$
• C

  $0.35\mathrm{ c}\mathrm{m}$
• Correct answer
  D

  $0.025 \mathrm{m}\mathrm{m}$

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