A solid copper sphere (density \[\rho \] and specific heat capacity c) of radius \[r\] at an initial temperature 200 K is suspended inside a chamber whose walls are at almost 0 K. The time required (in \[\mu s\]) for the temperature of the sphere to drop to 100 K is

• A. \[\frac{72}{7}\frac{r\rho c}{\sigma }\]
• B. \[\frac{7}{72}\frac{r\rho c}{\sigma }\]
• C. \[\frac{27}{7}\frac{r\rho c}{\sigma }\]
• D. \[\frac{7}{27}\frac{r\rho c}{\sigma }\]