Two large parallel planar walls are maintained at $1000\:K$ and $500\:K$. Parallel radiation shields are to be installed between the two walls. Assume that the emissivities of the walls and the shields are equal. If the melting temperature of the shields is $900\:K$, the maximum number of shield(s) that can be installed between the walls is (are)

1. $1$
2. $0$
3. $2$
4. $3$