如图所示，在光滑水平桌面上建立平面直角坐标系xOy．一质量为m的物块静止在坐标原点．现对物块施加沿x轴正方向的恒力F，作用时间为t；然后保持F大小不变，方向改为沿y轴负方向，作用时间也为t；再将力F大小不变，方向改为沿x轴负方向，作用时间仍为t．则此时（   ）

一个半圆图形，它的对称轴是这个圆的____。

Dear Jenny,
   Please take these things to your brother: his math book, baseball, notebook, CDs, and video tape. The math book is on the dresser. The baseball is under the bed. The notebook is on the bed. The CDs are in the drawer. The video tape is on the table. Thanks!
                                                                                                                                                                            Mom

仿照下面句式，写一个比喻句。

       同情是一缕春风，让枯秃的枝头绽出新绿。