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

1. $t{_1}/4$
2. $t{_1}/2$
3. $2{t_1}$
4. $4{t_1}$