18. As shown in the figure, there are three green semicircles above the line and three red semicircles below the line, $A B=B C=C D=D E=E F=F G$. An ant starts from point $A$ and heads to point $G$, the ratio of the distance it travels along the red curve to the green curve is ( ).
A. $1: 2$
B. $2: 3$
C. $1: 1$
D. $2: 1$
E. $3: 2$
$\mathrm{C}$