到中国去的平信要靠船运。而船的航班是不定期的。
Ordinary mail to China goes by boat and the sailings are irregular.
不定期票可以很好地利用航班资源，提高航空公司收益。
Aperiodic ticket can make use of flight's seats which bring out revenue for airline.
不定期票可以很好地利用航班资源，提高航空公司收益。
Aperiodic ticket can make use of flight's seats which bring out revenue for airline.
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