阅读下面一首古诗，完成小题。
                                                                                                                                          雁门太守行
                                                                                                                                               李贺
                                                                                                                      黑云压城城欲摧，甲光向日金鳞开。
                                                                                                                      角声满天秋色里，塞上燕脂凝夜紫。
                                                                                                                      半卷红旗临易水，霜重鼓寒声不起。
                                                                                                                      报君黄金台上意，提携玉龙为君死。

如图，矩形ABCD中，AB=2，AD=4，AC的垂直平分线EF交AD于点E、交BC于点F，则EF=____．

A，B，C，D四位同学到李爷爷家做客，李爷爷兴致勃勃的告诉他们，今年又是大丰收。种的萝卜、藕最大的重约5千克，冬瓜最重的二十几千克，四位同学就此发表了各自的意见，请你判断出正确的结论（    ）