下列说法正确的有（ ）
①0是最小的正数；
②任意一个正数，前面加上一个“﹣”号，就是一个负数；
③大于0的数是正数；
④字母a既是正数，又是负数．
从方框中选择最佳选项完成对话, 有两项多余。
A. Are these your parents?
B. Who is the boy on the left?
C. What’s your father’s job?
D. What’s this?
E. Is your mother a teacher?
F. Is this beautiful girl your sister?
G. What’s your father’s name?
Li Lei—L; Mary—M
L: Good morning, Mary.
M: Good morning, Li Lei.
L: ____
M: It's a photo of my family.
L: Oh. ____
M: Yes, she is. Her name is Lily.
M: Yes, they are.
M: He is a doctor.
M: No, she is a nurse.
根据拼音写出相应的词语。
下列各点中，在函数y=－3x的图象上的是( )
2009年1月，我国正式进入移动通信的3G时代．移动通信实际上是利用____传递信号的．某手机发射的信号频率是1000MHz，该信号的波长为____m．
