$8 \cdot 8$ Let $2^{x}=8^{y+1}$ and $9^{y}=3^{x-9}$, then the value of $x+y$ is
(A) 18.
(B) 21.
(C) 24.
(D) 27.
(E) 30.
(15th American High School Mathematics Examination, 1964)
[Solution] Given $2^{x}=8^{y+1}=2^{3(y+1)}$, and $3^{x-9}=9^{y}=3^{2 y}$, then $\left\{\begin{array}{l}x=3(y+1), \\ x-9=2 y,\end{array}\right.$ Solving, we get $\left\{\begin{array}{l}x=21, \\ y=6 .\end{array}\right.$ Therefore, the answer is $(D)$.