5. In trapezoid $ABCD$, $AB \parallel CD, AB=3CD, E$ is the midpoint of diagonal $AC$, and line $BE$ intersects $AD$ at $F$. Then the value of $AF: FD$ is ( ).
(A) 2
(B) $\frac{5}{2}$
(C) $\frac{3}{2}$
(D) 1
5. C.

Draw $C G / / B F$ intersecting the extension of $A D$ at $G$ (as shown in the figure). From $A B$ $/ / C D, C G / / B F$, we get $\triangle A B F \backsim$ $\triangle D C G$, thus $A F: D G=A B$ ～ $D C=3: 1$. Also, by $C G / / B F$, $F D=\varepsilon D C$, hence $A F: F l=-32=\frac{3}{2}$.