若方程组$\left\{\begin{array}{c}ax+y=5\\ x+by=-1\end{array}\right.$的解为$\left\{\begin{array}{c}x=2\\ y=1\end{array}\right.$，则点P（a，b）在第 ____象限．

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阅读下列内容，从每小题所给的A、B、C、D四个选项中选出一个最佳答案。

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