如图，在Rt△ABC中，∠BAC=90°，AB=AC，直角∠EPF的顶点P是BC的中点，将∠EPF绕顶点P旋转，两边PE，PF分别交AB，AC于点E，F．下列四个结论：

①AE=CF；②△PEF是等腰直角三角形；③EF=AP；④S_{四边形AEPF}=$\frac{1}{2}$S_△ABC．

在∠EPF旋转过程中，上述四个结论始终正确的有（　　）

    I like school days. I can learn many things from school. We have four classes in the morning and two in the afternoon. I like weekends, too. We don't go to school on Saturdays and Sundays. I often play ping-pong and play basketball on Saturday morning. I often play the violin (小提琴) on Saturday afternoon. I often do homework and play football on Sunday morning. I often go to the park on Sunday afternoon.

测量长度的基本工具是 ____，在国际单位制中，长度的主单位是 ____