8.25 Let $a, b$ be positive constants not equal to 1. Then the number of real numbers $x$ that satisfy the equation $\left(\log _{a} x\right)\left(\log _{b} x\right)$ $=\log _{a} b$ is
(A) 0.
(B) 1.
(C) 2.
(D) An integer greater than 2.
(E) Not a finite number.
(16th American High School Mathematics Examination, 1965)