Don’t forget to turn on your radio at 7 p.m. this evening. We’ll be back then.A. in the a

问题补充：

Don’t forget to turn on your radio at 7 p.m. this evening. We’ll be back then.

A. in the air

B. by air

C. on the air

D. off the air

答案：

A. in the air

B. by air

(答案→)C. on the air

D. off the air

解析：这题考查介词短语，in the air在空中，by air乘飞机，on the air在播放 ，句意是：别忘了今晚7点打开收音机，那时我们将回来播放。选C。