$8 \cdot 18$ If $\log _{2 x} 216=x$, where $x$ is a real number, then $x$ is
(A) an integer that is neither a perfect square nor a perfect cube.
(B) a rational number that is neither a perfect square, nor a perfect cube, nor an integer.
(C) an irrational number.
(D) a perfect square.
(E) a perfect cube.
(11th American High School Mathematics Examination, 1960)
［Solution］From $\log _{2 x} 216=x$, we have $216=(2 x)^{x}$, 
and $216=2^{3} \cdot 3^{3}$, so $(2 \cdot 3)^{3}=(2 x)^{x}$, thus $\quad x=3$. Therefore, the answer is $(A)$.