The diagram shows a star $S$ and the starting positions of three planets, $X$, $Y$ and $Z$. Each planet moves clockwise in a circular orbit around $S$. The time for one orbit of $Y$ is three times the time needed for one orbit of $X$. The time for one orbit of $Z$ is twice the time needed for one orbit of $Y$. From the starting arrangement, which diagram shows the positions of the planets after $X$ has completed one full orbit?