$A$ $U$-shaped wire is dipped in a soap solution and removed. The thin soap film formed between the wire and the light slider supports a weight of $1.5 \times 10^{-2} \; N$ (which includes the small weight of the slider). The length of the slider is $30 \; cm$. What is the surface tension of the film?

• A
  $6.32 \times 10^{-3} \; N m^{-1}$
• B
  $5.25 \times 10^{-4} \; N m^{-1}$
• C
  $6.8 \times 10^{-3} \; N m^{-1}$
• D
  $2.5 \times 10^{-2} \; N m^{-1}$