117 Let the geometric sequence $\left\{z_{n}\right\}$ be such that $z_{1}=1, z_{2}=a+b i, z_{3}=b+a i(a, b \in \mathbf{R}, a>0)$, then the smallest $n$ that satisfies $z_{1}+z_{2}+\cdots+z_{n}=0$ is
A. 6
B. 12
C. 18
D. 24