Let \( C \) be the locus of the mirror image of a point on the parabola \( y^{2}=4x \) with respect to the line \( y=x \). Then the equation of tangent to \( C \) at \( P(2,1) \) is:
\( 2x+y=5 \)
\( x+3y=5 \)
\( x-y=1 \)
\( x+2y=4 \)