$18 \cdot 110$ As shown, $A B C D$ is an isosceles trapezoid, $A D$ $=B C=5, A B=4, D C=10$, point $C$ is on $D F$, and $B$ is the midpoint of the hypotenuse of the right triangle $\triangle D E F$. Then $C F$ equals
(A) 3.25 .
(B) 3.5 .
(C) 3.75 .
(D) 4.0 .
(E) 4.25 .
[Solution] Draw $B H \perp D F$, then $B H / / E F$. Since $A B C D$ is an isosceles trapezoid,
$$
\therefore \quad C H=\frac{1}{2}(D C-A B)=3,
$$

then
$$
D H=D C-C H=10-3=7 \text {. }
$$
Since $B$ is the midpoint of $D E$, and $\frac{D B}{D E}=\frac{D H}{D F}$,
$$
\therefore D F=14 \text {. }
$$

Then $C F=D F-C D=4$.
Therefore, the answer is $(D)$.