13. As shown in the figure, $C, D$ are two points on the line segment $A B$, $M, N$ are the midpoints of $A C, D B$ respectively. If $A B=10, C D=2$, then the length of $M N$ is ( ).
(A) 3
(B) 4
(C) 5
(D) 6
Answer: D.
Solution: Since $M, N$ are the midpoints of $A C, D B$ respectively, let $A M=x, D N=y$.

Then
$$
2 x+2 y=10-2=8,
$$

Therefore
$$
x+y=4 .
$$

So
$$
M N=x+2+y=4+2=6 \text {. }
$$

Hence, the answer is D.