13. In Rt $\triangle A B C$, $B C=3, A C=4, A B$ $=5$. If the triangle is folded so that point $A$ coincides with point $B$, then the length of the fold line is ( ).
(A) $1+\frac{\sqrt{2}}{2}$
(B) $\frac{15}{8}$
(C) $\frac{7}{4}$
(D) $\sqrt{3}$
(E) 2