<image>The points $A, B, C$ and $D$ are marked on a straight line in this order as shown in the diagram. We know that $A$ is $12 \mathrm{~cm}$ from $C$ and that $B$ is $18 \mathrm{~cm}$ from $D$. How far apart from each other are the midpoints of the line segments $A B$ and $C D$?
A. $6 \mathrm{~cm}$
B. $9 \mathrm{~cm}$
C. $12 \mathrm{~cm}$
D. $13 \mathrm{~cm}$
E. $15 \mathrm{~cm}$