在平面上有有限个长方形， 它们的边都与XY轴平行， 而且， 任意两个长方形，我们都能找出一条水平线或者垂直线同时穿过这2个长方形， 已知这些长方形互不相交， 证明： 可以画一个十字（一条水平线和一条垂直线）， 使得这个十字穿过所有的长方形。

Given a finite number of rectangles, the edges are all parallele to the x,y axis. Such that, any two of them, we can draw a vertical line or a horizontal line that cross both of them, they are all disjoint. Prove that, we can draw a cross (that is, a vertical line and a horizontal line), that joint all the rectangles.