羊和青草的结构层次的不同是青草没有( )
A:细胞
B:组织
C:器官
D:系统
(2016•十堰)- I shouted to you at the gate, but you didn't hear me.
- Really? I ____ a telephone call to my mother.
函数y=lgx+2x-1的定义域是____
毛衣不能产生热量,起到保温作用。( )