15. The length of the interval of solutions of the inequality $m \leq 8 x-1 \leq n$ is 25, then 6
( ).
(A) $n-m=25$.
(B) $n-m=199$.
(C) $n-m=200$.
(D) $n-m=201$.
(English-Chinese Mini Dictionary: length 长度; interval 间隔; inequality 不等式)
Translation: The length of the interval of solutions of the inequality $m \leq 8 x-1 \leq n$ is 25, then ().
(A) $n-m=25$.
(B) $n-m=199$.
(C) $n-m=200$.
(D) $n-m=201$.
Answer: C.
Solution: The solution to the inequality is
$$
\begin{array}{c}
\frac{m+1}{8} \leq x \leq \frac{n+1}{8}, \\
\frac{n+1}{8}-\frac{m+1}{8}=\frac{n-m}{8},
\end{array}
$$

The length of the interval is

From the given condition, we have
$$
\frac{n-m}{8}=25 \text {, }
$$

Therefore,
$$
n-m=200 \text {. }
$$

So the answer is C.