根据短文内容，完成下列各小题。

    It's four thirty in the afternoon. There are many students on the playground. Jim and his friends are playing basketball. Li Lei and other boys are playing football. Miss Gao is watching the games. She is looking after the clothes, too. Where are the girls? They are over there. Some are running, others are jumping. The twins are on duty today. They are in the classroom. Lucy is cleaning the window, Lily is sweeping the floor. They are working hard. They are tired(疲倦的) but happy.

如图，四边形ABCD中，∠A=90°，AB=3$\sqrt{3}$，AD=3，点M，N分别为线段BC，AB上的动点（含端点，但点M不与点B重合），点E，F分别为DM，MN的中点，则EF长度的最大值为　____ 　．

如图，能正确反映地球自转的是（根据自转方向判断）（    ）