Two infinitely large parallel plates ($I$ and $II$) are held at temperatures $T_{I}$ and $T_{II}$ $(T_{I}>T_{II})$ respectively, and placed at a distance $2d$ apart in vacuum. An infinitely large flat radiation shield $(III)$ is placed in parallel in between $I$ and $II$. The emissivities of all the plates are equal. The ratio of the steady state radiative heat fluxes with and without the shield is :
- $0.5$
- $0.75$
- $0.25$
- $0$