6.10 The formula $N=8 \cdot 10^{8} \cdot x^{-\frac{3}{2}}$ represents the number of people in a group whose income exceeds $x$ yuan. The minimum income of the highest $800^{\circ}$ earners is at least
(A) $10^{4}$.
(B) $10^{6}$.
(C) $10^{8}$.
(D) $10^{12}$.
(E) $10^{16}$.
(11th American High School Mathematics Examination, 1960)
[Solution] From $800=8 \cdot 10^{8} \cdot x^{-\frac{3}{2}}$, we get $x=10^{4}$. Therefore, the answer is $(A)$.