Look, read and choose.（看图选择，将选项写在横线上）

1. -____ can you see?

    -I can see ____.

A. How many birds

B. How many kites

C. five

D. seven

2. -____ can you see?

  -----I can see ____.

A. How many cats

B. How many dogs

C. twelve

D. four

3. -____ can you see?

    -I can see ____.

A. How many cars

B. How many pens

C. ten

D. two

数据传输中的频率调制也叫频率键控，是用数字基带信号控制（）信号的频率f。