胚珠发育成的结构是（ ）
A：种子
B：种皮
C：果实
D：果皮
质量为m的A球以速率v与质量为3m的静止B球沿光滑水平面发生正碰，碰撞后A球速率为 v2 ，则B球速率可能为（ ）
We__________ listen to her CDs, because they are too bad.
— I called you yesterday evening, but there was no answer.
— Oh , I’m sorry I _________ dinner at my friend’ s home.
人体的组织中，具有收缩和舒张功能的是
