The bright image of $\frac{h}{x}$ is a time-scale graph, one of its axes of symmetry is the graph of which of the following direct proportion functions? ().
(A) $y=k x$
(B) $y=-k x$
(C) $y=|k| x$
(D) $y=-\frac{k}{|k|} x$
7. (D).

When $k>0$, the axis of symmetry is $y=-x$; when $k<0$, the axis of symmetry is $y=x$. Therefore, it is $y=-\frac{k}{|k|} x$.