A wooden box (open at the top) of thickness $0.5\:\text{cm}$, length $21\:\text{cm}$, width $11\:\text{cm}$ and height $6\:\text{cm}$ is painted on the inside. The expenses of painting are Rs. $70$. What is the rate of painting per square centimetres?
- $\text{Re}\;0.7$
- $\text{Re}\;0.5$
- $\text{Re}\;0.1$
- $\text{Re}\;0.2$