The population $p(t)$ at time $t$ of a certain mouse species satisfies the differential equation $\frac{dp(t)}{dt} = 0.5\; p(t) -450$. If $p(0) = 850$, then the time at which the population becomes zero is
( A ) $2\log 18$
( B ) $\log 9$
( C ) $\frac{1}{2}\log 18$
( D ) $\log$