18.105 The sum of the two base angles of a trapezoid is $90^{\circ}$, the upper base is 5, and the lower base is 11. Then the length of the line segment connecting the midpoints of the two bases is 
(A) 3.
(B) 4.
(C) 5.
(D) 6.
(Anhui Junior High School Mathematics Competition, 1996)
[Solution] Draw $C F / / A D$ intersecting $A B$ at $F$. According to the problem, $\angle F C B=90^{\circ}$.
Since $C E$ is the median of the hypotenuse of the right triangle $\mathrm{Rt} \triangle F C B$, then
$$
C E=\frac{1}{2} F B=\frac{1}{2} \cdot(11-5)=3 \text {. }
$$

Let $G$ and $H$ be the midpoints of the upper and lower bases $O C$ and $A B$ of the trapezoid, respectively.
Then $E H=B H-B E=C G=2.5$,
so $G H E C$ is a parallelogram.
Thus, $G H=C E=3$.
Therefore, the answer is $(A)$.