我怎样才能在 Latex 中的表格中仅绘制表格线的一部分，就像图片所示的那样？

Question

\documentclass[12 pt]{book}
\usepackage{amsmath}
\usepackage[table]{xcolor}
\begin{document}
    \begin{table}
            \caption{Definition of Linear Finite Difference Operators}\label{Table1}
        \centering
        \begin{tabular}{l l c l }
            %baraye naboudan khotute amoudi | ha ro hazf kardim {|c|c|c|c|}%
            \hline & \textcolor{red}{Operators} & \cellcolor{yellow}{Symbol} & Difference Representation  \\ \arrayrulecolor{green} \hline
            $(2.1.9a)$                                                                 & Forward Difference         & \textcolor{blue}{$\Delta$} & $\Delta u_r=u_{r+1}-u_r$   \\

            $(2.1.9b)$                                                                 & Backward Difference        & $\nabla $                  & $\nabla u_r=u_r - u_{r-1}$ \\ \arrayrulecolor{black}\hline
        \end{tabular}

    \end{table}

\end{document}