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