1. by air 乘飞机。
如:He went to Paris by air.
他乘飞机去了巴黎。
Which is faster, by air or by train?
坐飞机快还是坐火车快?
2. in the air 在空中;在流传中;渺茫。
如:Birds fly in the air.
鸟在空中飞。
The plan is quite (up) in the air.
计划还相当渺茫。
3. on the air . 在广播中。
如:I heard the news on the air.
我在广播中听到了这条消息。
What’s on the air this evening?
今晚有什么广播节目?