Bob________after his classmate in the classroom when his teacher________.[ ]

A. ran;turned up

B. was running;turned up

C. was running;would turn up

D. had run;would turn up

无线电通信所用的电磁波分几个波段，这是按[ ]

A．无线电波的波长分的

B．无线电波的频率分的

C．无线电波的波速分的

D．无线电波的波长、波速和频率分的