$17 \cdot 13$ As shown in the figure below, if $A D=A B=B C$, then the relationship between $\alpha$ and $\beta$ is
(A) $\alpha=2 \beta$.
(B) $\alpha+\beta=90^{\circ}$.
(C) $2 \alpha+\beta=180^{\circ}$.
(D) $3 \alpha-\beta=180^{\circ}$.
(Mathematics Competition for Junior High School, Jingzhou Region, Hubei Province, China, 1987)
[Solution] Given that $\angle B=\angle D$, and $\alpha=\beta+\angle D=\beta$ $+\angle B$,

and $\angle B=180^{\circ}-2 \alpha$, then $\alpha=\beta+\left(180^{\circ}-\right.$ $2 \alpha$.
That is, $3 \alpha-\beta=180^{\circ}$.
Therefore, the answer is $(D)$.