5. The smallest natural number $n$ for which the equation $\left[\frac{10^{n}}{x}\right]=2006$ has an integer solution $x$ lies in the interval $(\quad)$
A. $[1,6]$
B. $[7,12]$
C. $[13,18]$
D. $[19,24]$
5. B Since $\left[\frac{10^{n}}{x}\right]=2006$, then $2006 \leqslant \frac{10^{n}}{x}<2007$, so $\frac{10^{n}}{2006}<x \leqslant \frac{10^{n}}{2007}$, which means 0.00049850 $\cdots<x<0.00049825 \cdots$ Therefore, when $n \geqslant 7$, $\left(\frac{10^{n}}{2006}, \frac{10^{n}}{2007}\right]$ contains an integer, so the minimum value of $n$ is 7.