人体形态结构和生理功能的基本单位是（）
A.组织
B.细胞
C.核糖体
D.蛋白质
E.器官
参考答案：B
--- Everybody is playing outside. Can I join them, Mom?
--- _______ Not until you have finished practicing the piano.
A．Why not?
B．Will you?
C．I hope so
D．I’m afraid not.
设区域D：x2+y2≤a2，x≥0，则=().
=().