A screw gauge has 50 divisions on its circular scale. The circular scale is 4 units ahead of the pitch scale marking, prior to use. Upon one complete rotation of the circular scale, a displacement of $0.5\mathrm{mm}$ is noticed on the pitch scale. The nature of zero error involved and the lest count of the screw gauge, are respectively:

• A

  Negative, $2\mu \mathrm{m}$
• Correct answer
  B

  Positive $10\mu \mathrm{m}$
• C

  Positive $0.1m\mathrm{m}$
• D

  Positive, $0.1\mu m$

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