4. The equation of the curve symmetric to the curve $f(x, y)=0$ with respect to the line $x-y-3=0$ is
A. $f(y+3, x)=0$
B. $f(x-3, y)=0$
C. $f(y+3, x-3)=0$
D. $f(y-3, x+2)=0$
4. C Hint: Let $(x, y)$ be any point on the desired curve, then the symmetric point of $(x, y)$ with respect to $x-y-3=0$ is $(y+3, x-3)$. This point should lie on the given curve, hence $f(y+3, x-3)=0$.