Population of a town at time t is given by the differential equation ${dP(t)\over dt}=(0.5)\ P(t)-450$. Also P(0) = 850 find the time when population of town becomes zero.

(1) $\ln9$

(2) $3\ln4$

(3) $2\ln18$

(4) $\ln18$

(1) $\ln9$

(2) $3\ln4$

(3) $2\ln18$

(4) $\ln18$