A thin liquid film formed between a U-shaped wire and a light slider supports a weight of $1.5\times 10^{-2}\text{N}$. The length of the slider is $30\text{cm}$ and its weight is negligible. The surface tension of the liquid film is:

1. A
   0.0125 , \text{Nm}^{-1}
2. B
   0.1 , \text{Nm}^{-1}
3. C
   0.05 , \text{Nm}^{-1}
4. D
   0.025 , \text{Nm}^{-1}