$7 \cdot 85$ When the graph of the function $y=f^{-1}(x)$ is rotated $90^{\circ}$ counterclockwise about the origin in the coordinate plane, the function of the resulting graph is
(A) $y=-f(x)$.
(B) $y=f(-x)$.
(C) $y=-f^{-1}(x)$.
(D) $y=-f(-x)$.
(2nd "Hope Cup" National Mathematics Invitational Competition, 1991)
［Solution］Consider the following two consecutive graphing steps:
(1) Make the graph of $y=f^{-1}(x)$ symmetric about the $x$-axis;
(2) Make the graph of the above result symmetric about the line $y=x$.
This is the graph of the desired function. Thus, the desired function is $y=f(-x)$. Therefore, the correct choice is $(B)$.