In a Van-de-Graff type generator, a spherical metal shell is to be a \(15 \times 10^6 ~\text {V}\) electrode. The dielectric strength of the gas surrounding the electrode is $\mathrm{}$\(5 \times 10^7 ~\text {V/m}.\) What is the minimum radius of the spherical shell required?

1. \(20~\text{cm}\)
2. \(30~\text{cm}\)
3. \(25~\text{cm}\)
4. \(35~\text{cm}\)