在△ABC中，AB=AC，点D在边BC所在的直线上，过点D作DF∥AC交直线AB于点F，DE∥AB交直线AC于点E．

（1）当点D在边BC上时，如图①，求证：DE+DF=AC．

（2）当点D在边BC的延长线上时，如图②；当点D在边BC的反向延长线上时，如图③，请分别写出图②、图③中DE，DF，AC之间的数量关系，不需要证明．

（3）若AC=6，DE=4，则DF的值。

《预防未成年人犯罪法》设专章规定了对未成年人严重不良行为的矫正。这些“严重不良行为”包括（     ）

①学习成绩差                    ②纠集他人寻衅滋事

③旷课、夜不归宿            ④吸食毒品

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    I have a good friend. 1 Chinese name is Da Shan and his 2 name is Mark Rowswell. He 3 from Canada. He can 4 English and French (法语). And he speaks Chinese very 5. He likes China very much. Now he teaches English 6 Beijing. Da Shan 7 a big nose, big eyes 8 short brown hair. He has a happy family. His wife (妻子) is from Sichuan, China. They 9 two children. Da Shan's father and mother are in Canada. But 10 like China, too. And they often come to

China.