$6 \cdot 12$ If $10^{2 x}=25$, then $10^{1-x}$ equals
(A) 5 .
(B) 2 .
(C) $\frac{1}{5}$.
(D) $\frac{1}{50}$.
(China Partial Provinces and Cities Junior High School Mathematics Correspondence Competition, 1988)
[Solution] From $10^{2 x}=25$, we get $10^{x}=5$.
Therefore, $10^{(1-x)}=10 \cdot 10^{-x}=10 \cdot\left(10^{x}\right)^{-1}=10 \cdot 5^{-1}=2$. Hence, the answer is $(B)$.