A:
B:
C:
D:
痰是气管分泌的黏液和黏液粘住的灰尘、细菌等。
电磁波家族有无线电波、红外线、可见光、紫外线等,它们的____相同(选填“频率”、“波长”或“波速”),它们都在真空中传播,电台发射的某种电磁波波长为3m,其频率是____Hz.
把正确对话的代表字母写在横线上。
①Here you are, Grandma. ____ A. They are in my bag.
②What do you need? ____ B. I need some orange juice.
③Where are my rulers? ____ C. Great!
④Let's have lunch, Sally. ____ D. Thank you, Sally.