A fisherman illegally introduces some fish into a lake, and they quickly begin to take over. The growth of the population of this new species is modeled by P(x)=a0(2)x where x is the time in weeks following the introduction of the species and a0 is the initial number of fish added to the lake. If the fisherman introduced 3 fish initially. How many fish are there after 4 weeks?
A. 162
B. 24
C. 48
D. 1,296