A spherical storage vessel is quarter-filled with toluene. The diameter of the vent at the top of the vessel is $1/20^{th}$ of the diameter of the vessel. Under the steady state condition, the diffusive flux of toluene is maximum at

1. the surface of the liquid
2. the mid-plane of the vessel
3. the vent
4. a distance $20$ times the diameter of the vent away from the vent