A hollow metallic sphere of radius $r$ is kept at potential of $1$ $Volt$. The total electric flux coming out of the concentric spherical surface of radius $R ( > r)$ is
1. $4\pi \epsilon _0r$
2. $4\pi \epsilon _0r^2$
3. $4\pi \epsilon _0R$
4. $4\pi \epsilon _0R^2$