Projecting contour on the floor in a 3d plot

Question

The contour is drawn on the top surface, I'm trying to plot the contour on the floor (A-B).

\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{width=10cm, compat=1.9}

\begin{document}

\begin{tikzpicture} \begin{axis}
    [
    xlabel=$A$,
    ylabel=$B$,
    zlabel=$f_0$,
    grid=major,
    ]


\addplot3[
    surf,
    opacity=0.4,
    ] {4*x + 5*y};

\addplot3 [
contour gnuplot, samples=21]
        {4*x + 5*y};

\end{axis} \end{tikzpicture}

\end{document}

Answer 1

\documentclass[tikz,border=3.14mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{width=10cm, compat=1.9}
\begin{document}
\begin{tikzpicture} 
 \begin{axis}[
     xlabel=$A$,
     ylabel=$B$,
     zlabel=$f_0$,
     grid=major,zmax=55,zmin=-55
     ]


 \addplot3[
     surf,
     opacity=0.4,
     ] {4*x + 5*y};

 \addplot3 [
 contour gnuplot, samples=21,z filter/.code={\def\pgfmathresult{-54.9}}]
         {4*x + 5*y};

 \end{axis} 
\end{tikzpicture}

\end{document}