25-13 In the same Cartesian coordinate system, there are infinitely many lines with the expression $y=k x+b$ (where $k, b$ are real numbers, and $k \neq 0$). Among these lines, no matter how they are selected, it is necessary to ensure that there are two lines passing through exactly the same quadrants. What is the minimum number of lines that need to be selected?
(A) 4 lines.
(B) 5 lines.
(C) 6 lines.
(D) 7 lines.
(China Guangzhou, Wuhan, Fuzhou, and other five cities Junior High School Mathematics League, 1992)
[Solution] According to the quadrants in which the lines are located, the lines on the plane can be divided into the following six categories:
$$
k>0,\left\{\begin{array}{l}
b>0, \\
b=0, \\
b<0 ;
\end{array}\right.\right.
$$
$$
k<0,\left\{\begin{array}{l}
b>0, \\
b=0, \\
b<0 .
\end{array}\right.\right.
$$

To ensure that there are two lines passing through exactly the same quadrants, at least 7 lines need to be drawn. Therefore, the answer is (D).