\begin{figure}[htbp]\centering
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\node[anchor=west,align=left] at (3250,900) {圆域：源位置先验\\蓝点：31个检测站};
\node[anchor=west,align=left] at (3250,-700) {部分站点在圆域外\\用于接收向外发射的源};
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\caption{第四问集合覆盖模型得到的31个检测站及源位置区域。}\end{figure}