Answer
$\overline{AB}$ is congruent to $\overline{CD}$
Work Step by Step
Since B is the midpoint of $\overline{AC}$, segment AB is congruent to segment BC. In addition, since C is the midpoint of $\overline{BD}$, segment BC is congruent to segment CD. Using the Transitive Property of Equality. If $\overline{AB}$ is congruent to $\overline{BC}$, and $\overline{BC}$ is congruent to $\overline{CD}$, then $\overline{AB}$ is congruent to $\overline{CD}$.
