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A solid sphere with an initial temperature T${_1}$ is immersed in a large thermal reservoir of temperature T${_0}$. The sphere reaches a steady temperature after a certain time t${_1}$. If the radius of the sphere is doubled, the time required to reach steady-state will be

- t${_1}$/4
- t${_1}$/2
- 2${t_1}$
- 4${t_1}$