The surface tension of liquid is 0.5 N/m. If a film is held on a ring of area 0.02 m², its surface energy is

• A

  $5\times {10}^{-2}\text{ joule }$

• B

  $2.0\times {10}^{-2}\text{ joule }$

• C

  $4\times {10}^{-4}\text{ joule }$

• D

  $0.8\times {10}^{-1}\text{ joule }$