A metal sheet $4 \mathrm{m}$ on a side and of negligible thickness is attached to a balance and inserted into container fluid. The balance to which metal sheet is attached reads $0.50 \mathrm{N}$ and the contact angle is found to be zero. A small amount of oil is then spread over the metal sheet. The contact angle now becomes $180°$ and the balance now reads $0.49 \mathrm{N}.$ The surface tension of the fluid is

• Correct answer
  A

  $6.25\times {10}^{-3} \mathrm{N} {\mathrm{m}}^{-1}$
• B

  $1.25\times {10}^{-1} \mathrm{N} {\mathrm{m}}^{-1}$
• C

  $4.25\times {10}^{-2} \mathrm{N} {\mathrm{m}}^{-1}$
• D

  $0.1 \mathrm{N} {\mathrm{m}}^{-1}$

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