5. In trapezoid $A B C D$, $A D / / B C, \angle B=30^{\circ}, \angle C$ $=60^{\circ}, E, M, F, N$ are the midpoints of $A B, B C, C D, D A$ respectively. Given that $B C=7, M N=3$. Then $E F$ is ().
(A) 4
(B) $4 \frac{1}{2}$
(C) 0
(1) 6