21. The surface tension of liquid is 0.5 N/m. If a film is held on a

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 × 10-2 joule                             (b)  2.0 × 10-2 joule (c)  4 × 10-4 joule                     (d)  …