Answer:
[tex]\large\boxed{C.\ -\dfrac{1}{3}}[/tex]
Step-by-step explanation:
[tex]a_1,\ a_2,\ a_3,\ ...,\ a_n-\text{geometric sequence}\\\\r=\dfrac{a_2}{a_1}=\dfrac{a_3}{a_2}=\dfrac{a_4}{a_3}=...=\dfrac{a_n}{a_{n-1}}-\text{common ratio}\\\\\\\text{We have}\\\\a_1=18,\ a_2=-6,\ a_3=2,\ a_4=-\dfrac{2}{3},\ ...\\\\\dfrac{a_2}{a_1}=\dfrac{-6}{18}=-\dfrac{6:6}{18:6}=-\dfrac{1}{3}\\\\\dfrac{a_3}{a_2}=\dfrac{2}{-6}=-\dfrac{2:2}{6:2}=-\dfrac{1}{3}\\\\\dfrac{a_4}{a_3}=\dfrac{-\frac{2}{3}}{2}=-\dfrac{2}{3}\cdot\dfrac{1}{2}=-\dfrac{1}{3}[/tex]
