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. 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?