CN-LEAGUE 1996 Problem 2

For geometrical sequence $(a_n)$ , the first term $a_1=1536$ , common ratio $q=-\frac{1}{2}$ . Let $\pi_n=\prod_{i=1}^n a_i$ , so the lagerest one in $(\pi_n)$ is $\text{(A)} \pi_9 \text{(B)} \pi_{11} \text{(C)} \pi_{12} \text{(D)} \pi_{13}$