4. (Winston, 4ed) Determine the optimal strategies for each player. \begin{tabular}{|cccccc|} \hline5 & 10 & 1 & 10 & 2 & 1 \\ 1 & 2 & 10 & 7 & 5 & 20 \\ 2 & 7 & 5 & 10 & 10 & 7 \\ 7 & 20 & 1 & 1 & 1 & 2 \\ 20 & 7 & 10 & 7 & 1 & 10 \\ \hline \end{tabular}