8. As shown in Figure 5, in rhombus $A B C D$, $\angle A B C=120^{\circ}$, $F$ is the midpoint of $D C$, and the extension of $A F$ intersects the extension of $B C$ at point $E$. Then the measure of the acute angle formed by lines $B F$ and $D E$ is ( ).
(A) $30^{\circ}$
(B) $40^{\circ}$
(C) $50^{\circ}$
(D) $60^{\circ}$
8.D.

Extend $B F$ to intersect $D E$ at point $M$, and connect $B D$. Then $\triangle B C D$ is an equilateral triangle, $F$ is the midpoint of $C D$, and $B F$ bisects $\angle C B D$. Therefore, $\angle M B C=30^{\circ}$.

Since $F$ is the midpoint of $C D$ and $A D / / C E$, $\triangle A D F$ and $\triangle E C F$ are centrally symmetric about $F$. Thus, $C E = A D = C D$. Hence, $\angle C E M=30^{\circ}, \angle D M F=60^{\circ}$.